A seemingly impossible crime. The standard example being that of a murder victim found in a room with only a single door, securely locked from the inside. Can be the basis for a single plot, or an entire show. A well-designed Locked Room Mystery provides pleasure from trying to figure out the puzzle before it is revealed, from moments of dawning realisation, and from a satisfyingly logical solution. A poorly designed Locked Room Mystery only provides a feeling of having been cheated. Contrary to the name, Locked Room Mysteries don't necessarily have to be murders or take place in locked rooms, just to be crimes that seem to be impossible at first glance (e.g. contemplating how it's possible for someone to travel from one part of the island to another within minutes). The question of who is rarely as interesting in this kind of plot as how.
Originally from crime fiction, John Dickson Carr being an acknowledged master. It is noteworthy that Edgar Allan Poe's short story The Murders in the Rue Morgue, widely considered to be the first detective story, involves a Locked Room Mystery.
Appears on television in a number of forms. The relatively pure form as a sub-genre of crime television (e.g., Monk, Jonathan Creek) where the puzzle is eventually unraveled by an eccentric protagonist using subtle clues and pure reason.
The part of the show where the solution to the mystery is explained is The Summation. One common trick used is the Time-Delayed Death - for others see the Analysis page. Expect the police to have found that The Key Is Behind the Lock when the crime scene was discovered. If a murder victim turns up someplace that's just so weird it's hard to explain, that's Body in a Breadbox.
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Anime and Manga
One of the Detective Cilan episodes of Pokémon: The case of the missing jewels aboard their ship. A woman's Liepard's Eye jewel is stolen. Despite the door being locked and the outside being guarded by a Watchog, and the jewel under lock and key, the power goes off and the Liepard's Eye is stolen. Several clues point to Red Herrings, but Cilan eventually covers the truth: the man Ash battled in the tournament earlier in the episode purposely used an Electric type in their match to short circuit the power to cause a distraction, and had his Vanillite sneak into the room through a vent, make a key out of ice, and steal the jewel. The perp's role in the tournament would also serve as an alibi, since he'd be too busy battling Ash to steal the gem.
Gosick starts off in classic style, with the old lady shot through the eye in her locked room. Although it's a fairly simple mystery, this serves as a springboard for a whole ship-ton of intrigue in the next episode.
Although it is not a crime show, the anime Spiral has a number of locked room mysteries that the protagonist must solve, including one literal locked room murder.
The Suzumiya Haruhi two part episode "Remote Island Syndrome" (and the corresponding chapter in one of the light novels) has an example of this inside a locked room on an island hit by a terrible storm.
Detective Conan frequently uses these, though it also plays with this trope, as Conan solves a case when he realizes that a man died the moment his wife checked on him, but the rest of the room was set up to make it look like a locked room mystery.
One of these came up in Black Butler. Turns out to be a prank by the Queen and Ceil playing Chessmaster to deal with a target of his
Golgo 13, "The Serizawa Family Murders". Two witnesses (brother and sister) to an old murder agree to a meeting in a hotel room guarded — but not under surveillance — by the police. They go in, but when the police storm the room after several hours, only the brother is in the room, denying that he ever saw his sister that evening. Despite being the prime suspect, they have to release him because they never find even a trace of the sister's body. "Down the toilet, one by one."
Lampshaded in Domu, where the detectives have to investigate a series of suicides at one apartment complex. One man jumps off the roof to his death, despite the lock of the door to get onto the roof being rusted shut for years on end. The younger policeman points out how ridiculous this "sort of... locked roof mystery, right?" is. Too bad science can't explain it.
One of these pops up in Franken Fran to a group of former patients of Fran, with Okita specifically calling it by the trope name. The solution is a lot squickier than some of the other answers. The patients did it to themselves so Fran can operate on them.
In the Ace Attorney manga, Turnabout Showtime is referred to as "the world's smallest locked-room murder", when Flip Chambers is somehow fatally stabbed inside his Sparklestar costume, which cannot hold anything in its hands and is impossible to open by the wearer (although it comes to light that there are workarounds for that, such as catching the zipper on a piece if wire, or wearing the costume backwards).
Inverted on Bokurano. All kids die in front of the surviving cast on Zearth's cockpit except for Waku, who died on Zearth's shoulder, Kako, who was murdered, and Ushiro, who brawled alone in the anime. Of course, nobody except the kids and the higher-ups on the military know that. Then again, Daiichi ask Koyemshi to hide his corpse as to not let his siblings know he's dead.
The Sword Art Online short story "Murder Case in the Area" is a form of locked room mystery involving players in an MMO where Your Mind Makes It Real being PKed inside a town, where game mechanics make it impossible to be attacked or injured. Turns out that they're actually faked suicides, with players stabbing themselves with weak weapons, then pretending to be attacked after some time and teleporting out to simulate game deaths. These players thus succeed in Gaslighting a former member of their group into confessing to the murder of a fourth.
The 2003 Astro Boy show has an episode where notorious gangster Skunk Kusai is suspected of several bank heists where the vaults were tunneled into but examination reveals the holes to have been dug from inside the vaults and the doors were never tampered with. Turns out he had kidnapped the inventor of a powerful electromagnetic device and used it to rearrange all the gold in the vault into the shape of a giant robot centipede, making the loot dig itself out and deliver itself to him.
Donald Duck and especially Mickey Mouse comics occasionally feature some versions. Unfortunately, it's poorly executed much of the time; if it seems the crime could only have been committed by a thief who could turn invisible, it's likely that's exactly what they did somehow, using a gadget or magic spell. However, there do exist some decent stories that feature an actual mystery of this sort as well.
The central case in Bookhunter involves three concentric locked room mysteries: The thief entered a locked library, removed a book from a locked safe, and carried the book out past the alarm checkpoints—leaving so little evidence that the theft wasn't noticed until weeks later.
There's one featured in Batman #700, where the significantly aged corpse of Carter Nichols is found dead in his basement. Batman and Robin are unable to solve the case, but it is later shown that the present Carter Nichols went forward in time and killed his future self, causing his dead future self to return to the present. It's a bit confusing.
A murder committed in a locked room would have been part of the plot in Alan Moore's never-written Deconstruction Crossover, The Twilight Of The Superheroes. Of course, it's easier to do in a world with superheroes, including some who can turn invisible and intangible.
Major plot point in The Shawshank Redemption, where the protagonist spends years digging a getaway tunnel through the wall of his prison cell. His escape leaves the wardens dumbfounded until they find the tunnel entrance behind a movie poster stuck to the wall.
The Film of the Book for Agatha Christie's Evil Under the Sun involves a bunch of suspects, all with airtight alibis. Naturally, the solution involves a bath, a bottle, a watch, and a bathing cap.
This was done, by the way, because the original book used Beneath Suspicion as a trope, and the filmmakers probably figured this would be a lot harder to hide in a movie than in a book.
To some extent, I, Robot fits this trope: Dr. Alfred J. Lanning's death looks like an open-and-shut suicide because the door to his room was locked. Spooner, of course, thinks otherwise. This is correct, incorrect, and a major Batman Gambit / Thanatos Gambit on the part of the victim all at the same time.
Inverted in Law Abiding Citizen, in that Clyde is somehow pulling off elaborate murders while locked in solitary confinement.
Teddy Daniels is brought to Shutter Island to investigate a locked room mystery - in this case someone escaping from a locked and guarded room.
We don't learn how Eli escaped from his cell in The Book of Eli. The room wasn't locked but guarded.
Played with in Blood Simple, where the female lead disappears from a bathroom without a trace.
In "The Verdict" (1946) Sydney Greenstreet plays a Scotland Yard inspector forced to retire after his investigation sends an innocent man to the gallows. Soon after that the nephew of the victim in the first case is found stabbed to death in his bed in a locked room that Greenstreet had to force open. Actually the man was still alive (but drugged) and Greenstreet stabbed him to death after forcing open the door. He covered his actions by screaming that the man was already dead, causing the landlady to recoil in horror, missing his actions. The Inspector murdered the man for two reasons. First he realized that the man had murdered his aunt for her money and let an innocent man hang for it. Also he hoped that his arrogant successor at Scotland Yard would never solve the case, humbling him. In the end he had to confess to the crime to prevent another innocent man from being hanged for murder.
The original locked room mystery is "The Murders in the Rue Morgue", by Edgar Allan Poe. The story became the Trope Codifier for later detective murder mysteries.
The deuterocanonical Old Testament story Bel and the Dragon has similarities to a locked room mystery.
The most famous Sherlock Holmes locked room mystery is probably "The Adventure of the Speckled Band".
Arguably, "The Adventure of the Empty House" was more popular.
Parodied in the Discworld short story "Theatre of Cruelty", in which Vimes's Internal Monologue brings up the complaint that "wizards made locked room mysteries commonplace" (though there are never any actual examples of this happening in the series).
The ultimately non-fatal poisoning of Lord Vetinari in Feet of Clay is a completely fair locked room mystery. It has an interesting partial subversion in that the victim figures it out long before the "detective" does, but lets him do his job anyway.
Presumably the very elderly wizard who, early in the series, tried to avoid Death by sealing himself inside a completely impenetrable room, became a self-inflicted Locked Room Mystery when his colleagues noticed his absence. (He'd overlooked the necessity of air holes.)
Happens in the Finnegan Zwake series with the man in Finn and Stoppard's storage room (in Horizontal Man) and Professor Freaze in his tent (in Worm Tunnel).
Intentionally played to the point of absurdity in The Long Dark Tea-Time of the Soul by Douglas Adams. Dirk Gently finds that his newest client, a wealthy man who had hired him as a security guard, had his head severed and placed on an active record machine while awaiting Dirk. Of course, the door to the room was locked from the inside when the scene was initially discovered. The police analyze this as an elaborate suicide done simply to cause trouble. In fact, magic was involved, but Dirk concocts a ridiculous but just barely plausible explanation of how it could be a suicide rather than try to convince them that magic is real.
Occurs in Jeffrey Deaver's novel The Vanished Man, where the killer is seemingly able to escape from a locked room where one of his victims is found, as well as disappear into a small crowd.
Played with in Jasper Fforde's The Big Over Easy, a detective story using nursery rhyme characters. "The entire crime-fighting fraternity yesterday bade a tearful farewell to the last 'locked room' mystery at a large banquet held in its honor. The much-loved conceptual chestnut of mystery fiction for over a century had been unwell for many years and was finally discovered dead at 3:15 A.M. last Tuesday."
Then it turns out that the locked room mystery was murdered... in a locked room.
In her non-fiction book Sex Crimes, Alice Vachss (wife of crime writer Andrew Vachss) mentions a real life case that she prosecuted, where the accused was locked in his room every night by nuns, and so supposedly could not have committed the rape. In the end the jury decided that any youth in that position would have found a way out of his room long ago.
John Dickson Carr, the acknowledged master of this back in the golden age of crime fiction, provided all sorts of different ways to accomplish this. In his book The Hollow Man/The Three Coffins, series lead Dr. Gideon Fell actually gives a lecture on the different ways a locked room mystery can be created. If the detective is Fell, Henry Merrivale, or Henri Benicolin, there is an excellent chance you've got a locked room or impossible crime on your hands.
Randall Garrett used this trope often in his Lord Darcy stories, with the added twist that magic is real in Darcy's world. Magicians naturally become prime suspects in a Locked Room Mystery, yet Lord Darcy often works out a non-magical explanation, thus exonerating some innocent wizard of the crime.
"Death in the Dawntime", by F. Gwynplaine Macintyre was written specially for The Mammoth Book of Historical Detectives, and probably has the earliest setting ever for a detective story. It's a Sealed Cave mystery.
Stieg Larsson'sMen Who Hate Women/The Girl with the Dragon Tattoo, is about a journalist investigating a forty year old murder which is a Locked Room Mystery on an island.
It's a subversion, though. The girl in question did live on an island, and she did disappear without a trace during a time period when the only bridge to the mainland was closed off. However, her uncle later admits that the family didn't realise that her disappearance could have been against her will until the bridge was already open again, and by then she and the abductor could have been long gone.
Ellery Queen had a number of locked room mysteries, including The King Is Dead.
The eighth Ellery Queen mystery, The Chinese Orange Mystery, is a locked room mystery with exceedingly weird clues, including the fact that the murder victim is found with his clothes on backwards.
This is actually an unusual case, because it's not presented as a locked room mystery, the murder room having a perfectly accessible back door. In fact, presenting it as a locked room mystery would have actually given away the killer's identity immediately!
The Psych tie-in novel Mind over Magic centers on the case of a costumed magician who appears to dissolve into nothingness in a water tank before a crowd of spectators but never reappears after the illusion, with a dead body suddenly in the tank in the magician's place.
Amelia Bones's body was found in this fashion by the Muggle police in Harry Potter and the Half-Blood Prince. The police of course couldn't figure what happened, but the reader knew that she was a wizard killed by the Death Eaters.
One of the "girls" disappears from a third floor room in The Alienist.
C. Daly King's Obelists Fly High is a variant: the murder takes place on an airplane.
The novel The Mystery of the Yellow Room, by Gaston Leroux (better known as the author of The Phantom of the Opera), is a Locked Room Mystery that is also a perfect Fair Play Whodunnit. The twist solution to the mystery, while being completely unexpected to the unprepared reader, manages to be on reflection the only logically possible solution given the facts of the case.
One of the characters in the prologue to Roger Levy's Reckless Sleep is a fan of locked door mysteries, and has thought up a fairly elaborate one of his own which he gleefully shows off to an uninterested accomplice.
The protagonist of the short story "The Man Who Read John Dickson Carr" by William Brittain murdered his wealthy uncle and escaped from the room through the chimney in order to confuse the inevitable investigation. It might have worked if he'd remembered to lock the door.
In the first installment of the Max Liebermann Papers by Frank Tallis, A Death in Vienna, the murder is committed and discovered in an actual Locked Room scenario.
One Tarma and Kethry short story featured a locked room mystery, with all of the locals assuming that the victim's wife was the killer, because she was the only person other than the dead man who had a key to the room in question. The real killer actually killed the victim before the door had been locked, and used a clever device to bar the door from the inside after he left.
Many of the Simon Ark mysteries involve some kind of variation on the locked room mystery. The implication is usually that some sort of occult forces are involved. The reality inevitably turns out to be something much more mundane.
Rita Yarborough investigates one in the short story "Behind Locked and Bolted Door" in the collection More J.T.'s Ladies.
In the light novel "Another Note" all five of BB's murders occur in rooms locked from the inside, which confuses the detectives endlessly as, according to Naomi, this is usually done to make a murder look like a suicide, and these were all obvious homicides. At the end, it is revealed that BB set up the locked rooms to make a suicide look like a murder, effectively framing his death on himself.
As Ryuzaki points out, the mystery could be solved by assuming that the killer has a key to each room. Though that doesn't turn out to be the case; Ryuzaki had been trying to lead the investigation astray.
Elliott Roosevelt, son of FDR, wrote a series of detective novels which cast his mother in the role of the sleuth. One of these, Murder in the Oval Office, is about the murder of a Congressman in the Oval Office after working hours. The murderer uses a simple trick to lock the door behind him in a failed attempt to make it look like suicide.
The first book in the Obsidian And Blood trilogy, Servant of the Underworld, involves one of these. A priestess vanishes entirely from a locked and guarded compound. The room she vanishes from? Covered in blood. But there's no blood trail leading out, and no one in the compound saw what happened to her...
In the Nick Velvet short story "The Theft of the Overdue Library Book", Nick has to work out how Sandra Paris abducted someone from a men's room that had an angry guard dog outside its only exit.
Most of The Great Merlini's cases were some form of "impossible crime", with actual locked rooms common as well.
Live Action TV
D.I. Richard Poole faces one of these in the very first episode of Death in Paradise.
Two episodes in Season 1 "Squeeze" and "Tooms" epitomize this trope variant, with the MOTW managing to get at his victims in otherwise inaccessible places through his mutant ability to squeeze through tiny vents, windows, chimneys etc.
Jonathan Creek is a show built around various sealed room mysteries (which feature in 13 of its 26 episodes to date) and other seemingly impossible crimes. A few examples:
In "Danse Macabre", a murderer carries a hostage into a sealed room. When the door is opened, only the hostage remains. The solution: the murderer was the hostage, in disguise, carrying a dummy. The dummy was cut up and stashed in the corner.
In "The Scented Room", a painting vanishes from a locked room during a school visit. The solution: a schoolgirl used the confusion of the school trip to ensure she was locked in the room, then simply stashed the painting in a hollow door panel.
In "Jack in the Box", a man is found dead in his nuclear bomb shelter, locked from the inside. Suicide... except he was arthritic and couldn't hold a gun. The solution: it was murder-suicide - the murderer bricked himself into an unfinished wall and drugged himself to death.
In "The House of Monkeys", a scientist is found stabbed by a suit of samurai armor in his locked office. The solution: he was drugged by an Animal Wrongs Group, and while under its influence fell on the sword.
In "The Omega Man", the army seize what a prominent Ufologist claims is an alien's skeleton. While locked in a crate to take it back to base however, it vanishes. The solution: the Ufologist had made it himself from frozen mercury. Locked in the crate, it simply melted away.
In "Ghosts Forge", Jonathan's assistant vanishes from a locked room while investigating a mystery. The solution: to take him down a notch, she'd simply bribed a builder to plaster over a door in the room and hidden in there.
In "The Three Gamblers", the corpse of a dead criminal seems to have climbed the stairs from the cellar where the body was left and scratched at the door. The solution: flood waters moved the body, dumping it there when they receded.
Inverted in "No Trace of Tracy", where the prime suspect claims to have been handcuffed to a radiator in a room in his house at the time a girl was abducted. The solution: while unconscious he had been carried to a replica of that room
Monk doesn't usually have crimes that look totally impossible, just ones that look impossible for Monk's prime suspect to have committed. The prime suspect is always guilty anyway. Some episodes turned this Up to Eleven, with one killer who was in outer space at the time of the murder, and still turned out to be guilty, and another where a guy managed to commit murder while in a coma. In one case, a criminal even tried to avoid blame for ordering a murder by provoking a witness report that seemed to indicate he had done it personally, which actually was impossible. There was also a more traditional locked-room crime in an episode where the victim was a magazine publisher who died in an apparent accident while working out in a locked room. The killer was in the room below with a very powerful magnet, which he used to make the weight the victim was using fall on his windpipe and crush it. In another episode, a record producer is found shot inside a locked panic room which the police had to literally cut a hole in the door to enter, the only other living thing in the room being a Chimpanzee who was holding the gun when the police came in. The security man who built the room had a secret entrance.
One Crossing Jordan episode involved a man who was writing a book on vampires found dead in a locked room, drained of blood and with fang wounds in his neck. It turned out to be self-inflicted.
On Kamen Rider Agito, the hallmark of a killing by the Unknown is that it is, by all rights, impossible - drowning in the middle of a field, being buried deep underground without any signs of digging, being entombed inside of a tree, your internal organs being ripped apart without the skin being broken, or falling to your death in a place where there's nowhere to fall from.
A classic example is the Star Trek: The Original Series episode "Wolf in the Fold". A woman is murdered on a planet, and it at first seems obvious that Scotty is the killer; he's clutching the murder weapon. But things start to get weird; after Scotty claims he couldn't remember what happened, the ruler of the planet suggests consulting his wife, who's an empathy, and she gets strange visions of a great evil with a hatred of women, calling out several names like "Beratis", "Kesla" and "Redjac". Then the lights are cut, and she's killed too, again apparently by Scott. Eventually, the computer aboard The Enterprise discovers the truth, that "Redjac" is an incorporeal being that feeds on fear which has been behind several unsolved cases of serial killers on many worlds, including Jack the Ripper.
The Star Trek: Deep Space Nine episode "A Man Alone" is a locked room mystery, with the added twist that the only DNA found in the room is that of the victim. It turns out that the "victim" is actually the killer, and the body found is that of a clone.
The mysteries on Banacek were all 'impossible crime' thefts; including locked room mysteries.
The first murder in The Conditions of Great Detectives is one. Tenkaichi lists how every single locked-room murder is created and that they're so popular because they're the riskiest yet most brilliant trick to pull off, not to mention how utterly tiresome he finds them. The one in this episode was caused by the old building becoming deformed when it snowed during the night, meaning the unlocked door became stuck. So the murderer made it appear as though they had barred the door, so when the police broke the door down it looked like it was locked from the inside.
Ellery Queen: In "The Adventure of the Disappearing Dagger", Ellery must solve a locked room mystery that occurred five years earlier in order to solve the current murder.
Sherlock: In "The Sign of Three", a guardsman is stabbed inside a shower cubicle that is locked from the inside with no weapon inside the cubicle. As Sherlock puts it, they are looking for an invisible killer with an invisible knife who can pass through walls. The answer to the mystery eludes everyone, including Sherlock.
Los misterios de Laura: A guy drowns in sea water while travelling on a plane, with several witnesses around who didn't notice anything strange- namely, he didn't drink a single drop of water, and nobody touched him during the flight. One of the attendants poisoned his chewing gum, and then bribed the forensics to add water inside his lungs while working on his autopsy.
In Warhammer Fantasy, Deathmaster Snitch killed an Imperial noble that had locked himself into a tower with the only door being guarded by a large band of knights. The knights didn't notice anything until the following morning. In fact, no explanation is ever given for how he did it, making it even more of a mystery.
The second game has a case where your assistant is locked in an empty room with the victim, a gunshot is heard, and the door is opened to reveal her standing over the corpse with a smoking gun in her hand. Naturally, it now falls on you to prove her innocent in court.
Another case involves a man found dead in the middle of a snow-covered courtyard with only one set of footprints leading to the body... and an eyewitness who saw the killer leave the murder scene by flying over the rooftops.
And in Ace Attorney Investigations we get a double murder that comes this way.
In Apollo Justice: Ace Attorney there's a case where the killer has to have escaped from the room in the few seconds after Apollo and Ema heard the gunshots.
There are other cases such as "Bridge to the Turnabout" that seem pretty straightforward to begin with, until Phoenix finds something that points to the crime scene being somewhere else, leading to another day of investigation for both parties.
Dual Destinies features a locked room in case two. There were two keys to the room, one of which was found in the locked room itself after the murder by police, and another was taken by a witness when she unlocked the door and found the scene. The defendant was found unconscious in the room, after it was unlocked. Therefore it was impossible for a third person to get out the room after they killed the victim, otherwise the door wouldn't be locked like it was. It turns out that the real killer killed the victim, and placed the unconcious defendant inside another room, which was thought to be unopenable by anyone other than the victim. Then, he disguised himself as the defendant using a wrestling mask, which the witness could not take off, as per masked wrestling protocol. Finally, after the witness left, he opened the second locked room, retrieved the defendant, disposed of the mask, and then left the room.
An interesting inversion of this trope occurs in The Elder Scrolls IV: Oblivion during a side mission; You and five other people are locked into a mansion on the premise that hidden somewhere in the house is a chest full of gold. Whoever finds it gets to keep it. A fun little game between friends, right? But soon, people start turning up dead, one by one, and suspicions fly as to who the killer is. Now, here's the twist; This is a Dark Brotherhood, a.k.a. Assassin's mission. YOU are the killer. The other five people are all targets, the mission is to kill them without them knowing you are the killer, and just for giggles, you're holding the only key to the front door.
Oh, yeah, and there's no gold, either. Well, not for them, anyway.
Unfortunately, it's programmed so badly that they never notice that you're the murder even if you do it in front of them.
Trauma Team's Forensics mode features a stage with this trope's exact name.
Dr. Schrader's death in Professor Layton and the Diabolical Box is one of these. Layton concludes that even though the office was too high up to exit from the window, the culprit did so anyway — by tearing off a curtain and fashioning it into a makeshift rope.
In one chapter of Ghost Trick, Lynne's corpse is found alone, in a room with only one way out that leads to several police officers waiting on the next floor. Of course there's no real mystery to solve, Sissel just has to go four minutes back in time to find out what happened. She activated a Rube Goldberg Device murder machine, which was designed to kill anyone who turned on the lights and then destroy any evidence of its own existence.
Invoked repeatedly in Umineko no Naku Koro ni, where it forms the core of the argument that the culprit must be the Golden Witch Beatrice instead of a human. But Beatrice is knowledgeable about the classics and doesn't stop here: she will abundantly shower the hero (and the reader) with red truths showing the perfection of the closed room and thoroughly wipe out any theory Battler may attempt. And on top of that, showing him (and the reader) fantasy scenes full of demons, magical barriers and Laser Blades. All while laughing.Of course, the point of the game is to not be fooled and find non-magic explanations. Bonus points for the first twilight of the third game: six people are found dead in six separate closed rooms, each victim having the key to the next room near their corpse.
The fourth murder of Danganronpa is presented as one of these, with the victim alone in a room with the door jammed shut to the point that Naegi had to break open a window on the door to get it open. Eventually subverted in that there is no murderer. The victim committed suicide.
The fate of the Val'Dutan'vir Ill'haressnote Clan leader in Drowtales. When their fortress was suddenly attacked by demons and Nidraa'chal summoners, Val'Dutan'vir forces succeeded in defeating them and not letting a single one reach even close to their clan leader. When they checked on their Ill'haress, they found her killed by her own guardian summon, who had somehow been tainted and turned into a demon. To this day, the survivors have no idea how their Ill'haress' guardian was tainted when no one should have been anywhere near the room.
Reach's story (The Big Idea) in the Whateley Universe features one of these prominently. There's quite a bit of discussion about the ways in which one needs to examine such a mystery, before the final denouement happens more or less as expected.
If criminal investigators encounter a Locked Room Mystery in real life, the usual default assumption (barring physical evidence to the contrary) is that the victim locked themselves in the room before succumbing to a Time-Delayed Death. Given how rare it is for an Instant Death Bullet and/or Poison to occur outside of fiction, and how a threatened person is likely to put a barrier between themselves and an attacker if they can, this sort of unintended Locked Room Mystery happens too often in the real world to really count as a "mystery".
Arthur C. Clarke used a Locked Room Mystery in his descriptions of spatial dimensions: if two-dimensional beings had a "bank vault" sealed on all sides — i.e. a square — then a 3D thief could remove its contents via the third dimension, leaving the 2D police baffled.