As anyone who has worked in a library or used book store can tell you, people use the damnedest things as bookmarks. In fiction, this can extend to vital pieces of evidence.
This trope is when someone (most often a murder victim) uses something as an impromptu bookmark that later turns out a vital clue in solving the crime. Expect a scene where the detective is searching the victim's room, picks up the book to flick through it—perhaps wondering if the victim had been researching something relevant to the crimes—and either notices the bookmark or has it fall out.
Differs from Book Safe in that the concealment was not intentional, and the book has not been modified.
- In The Disappearance of Haruhi Suzumiya, Kyon returns to the Literature Club room the next day and finds The Fall of Hyperion, which was the same book Yuki used to reveal her true identity in the The Melancholy of Haruhi Suzumiya. Kyon opens the book and finds a bookmark that reads: "Program Run Condition: Collect the keys. Deadline: Two Days Later". Kyon is unsure about the message's significance and decides to heed Yuki's request to go to her apartment.
- In Twelve Moons and a Fortnight, remnants of Mo Xuanyus distinctive makeup are discovered between the pages of a book Nie Huaisang gives to Wei Wuxian. Wei Wuxian identifies the makeup by its scent, and deduces that it must have belonged to Mo Xuanyu because the books original owner was a doctor who would not have regularly worn makeup, and Mo Xuanyu was known for heavily powdering his face.
- In The Dry, Aaron is returning Karen's library books after her murder when he notices a newspaper article about himself with his phone number written at the top and the borrowing receipt with "GRANT?" written on the back wedged between the pages of one of the books. Both items prove to be important clues in solving the murders once he works out what they mean.
- Lo: A loner named Justin falls in love with a Manic Pixie Dream Girl named April. She is kidnapped by demons and leaves behind a book. Looking for any clue to save April, he finds a bookmark, showing how to summon a demon named Lo.
- Played with in the film adaptation of The Lords of Discipline. Set in a southern military academy during the 1960s, the protagonist is trying to help a black cadet deal with racial harassment. He advises the cadet that if he wants to contact him, there's a book in the library that's never been checked out in the history of the school, and he should leave a note for him in that book.
- In She Lover of Death, Columbine is Driven to Suicide (she survives that attempt, though) by the discovery of a message from Death Himself, which mysteriously appears in a book she tries to read. After examining the book, Erast Fandorin quickly figures out that this was a ploy by Prospero, who wrote the message in the book with invisible ink, then inserted a bookmark in such a way that a different set of chemicals on it would seep through to the target page and reveal the message after a couple days.
- One commonly-used plot in the "mini-mystery" genre (including Encyclopedia Brown) features a Bookmark Clue as the key to the case. A large or antique banknote (usually $100, but sometimes more Depending on the Writer) is stolen from someone who claims that they were using it as a bookmark and left it between two particular pages. After the detective interviews the suspects, they ultimately determine that the supposed "victim" engineered the theft to get others in trouble or claim insurance fraud. The bookmark's supposed location is the error, as the detective declares that the pages the crook named are on opposite sides of the same leaf of a book, rather than two different leaves—it wouldn't be possible for someone to leave any sort of bookmark "between" those page numbers. (Yes, Conviction by Contradiction is at play here, but why spoil a good mystery?)
- In Have His Carcase, the murdered man gave a document to his mistress, who used it as a bookmark and then forgot about it. It thus survived when he burned his papers and was later found by Wimsey.
- Colonel March of Scotland Yard: In "The Stolen Crime", the Victim of the Week is murdered when the killer douses her cigarettes in a Perfect Poison. The killer then enters her room and removes all her cigarettes and empties the ashtrays so there is no evidence of how the poison was administered. However, when March searches the room, he discovers that she had used a cigarette as a bookmark.
- CSI: In "Nesting Dolls", Warrick has finds a dictionary with certain romantic words highlighted. A set of camera booth photos featuring the Victim of the Week Svetlana and another guy is being used as a bookmark. Catherine tells Warrick that the guy in the pictures is not the victim's husband Andrew.
- Death in Paradise: Inverted in "Frappe Death Day" where DI Mooney becomes fixated on the fact that the Victim of the Week hadn't placed a bookmark in the heavy tome he was reading before he was shot. Typically for the show, this Absence of Evidence turns out to be a vital clue.
- 'Midsomer Murders: In "The Stitcher's Society", the SOCO team find an important clue—a newly signed lease—in a book in the first Victim of the Week's house.
- In Fahrenheit, the two detectives Carla and Tyler find a copy of The Tempest in a crime scene, at the table that Lucas (the killer) was sitting at. Tyler finds a piece of scrap paper inside that Lucas was using as a bookmark, and a friend of Carla's identifies the scrap paper as a printout from a financial institution, enabling the detectives to track down Lucas to his office.
- In Infocom's Witness, one potential clue is a receipt that Hong the butler carelessly used as a bookmark.
- Perhaps a literal example in the Adventures of Sonic the Hedgehog episode, "The Mystery of the Missing Hi-Tops". Sonic's iconic red sneakers go missing when Sonic stays at a hotel during a festival being held in his honor. On the day before the festival, a salesman named Harry the Hawker gives Sonic a complimentary bookmark. When Sonic interrogates Sonette, a Loony Fan of his, she admits that she stole something from him when she snuck into his hotel room the night before. It wasn't his sneakers, but it was the bookmark that Harry gave him. However, Sonic discovers that it wasn't the same bookmark, because the bookmark Harry gave him was blue, and the bookmark Sonette stole and returned to him was red. This is how Sonic deduces that Harry was the one who stole his sneakers, as during his heist, Harry accidentally dropped a red bookmark, and when he tried to pick it back up, he took the blue one by mistake. He left the red one behind, which Sonette stole.