A mysterious note from a stranger changes the plot in some way. Generally, it is shoved under or into something such as a locker, postbox, or a door. If the recipient sees the note drop on his mat and dives for the door, he will occasionally catch the messenger in the act of posting the note; however, the messenger usually leaves so fast it's as if he teleported.
Can be anything at all, for example:
- A clue from an anonymous source
- A note from a past or future self
- A revealing tape of some kind
- A threat of some kind
- Ransom demand
Not to be confused with Brown Note or any other sound related trope, although it can contain, or be a recording of any kind.
- In Ore-sama Teacher, both Mafuyu and Hayasaka receive notes in their lockers saying "360 degree warning". This presages the appearance of a new character, a wacky fake 'ninja' who is trying to destroy the Public Morals Club.
- The All Guardsmen Party's discount spaceship, the Occurrence Border, is covered with little yellow post-it notes bearing helpful messages such as "The gravity in this corridor is tilted 37 degrees to the left" or, all too often, "Do not ever touch this button." That's in realspace, though, once the ship lurches its way into the Warp, more notes start showing up in areas the party has already visited, with always helpful but disturbingly specific advice like "In case of bad dreams check Gellar Field integrity."
- In Harry Potter and the Methods of Rationality, one of the results of Harry's experiments with a Time Turner is a note from his future self reading only "DO NOT MESS WITH TIME"
- The Incredibles. The "note" is in the form of a digital tablet with a video message inviting Mr. Incredible to do secret superhero work.
- 1BR: After a day or two of staying at the apartment, Sarah receives a note that's slid under her door. It tells her that she's in violation of the building's "No Pets" policy, which has an additional notice written on it in red capital letters that more-or-less says "SOME TENANTS ARE ALLERGIC YOU SELFISH BITCH".
- Memento: The hero receives a few of these. Some of them from himself, since he suffers from anterograde amnesia.
- Murder by Death. While Sam Diamond and Miss Skeffington are in their room, they hear a ticking sound. A note is slipped under their door that says, "That ticking sound is a bomb that will go off in 30 seconds. Signed, The Murderer."
- In Sherlock Holmes and the Secret Weapon, Holmes receives an envelope that is supposed to contain a coded message from Dr. Tobel, containing vital information regarding the bomb sight. However, when he opens the envelope, he finds the coded message has been replaced by a taunting note from Professor Moriarty.
- In Unsane, a horror/thriller author receives one about a series of murders related to his latest book.
- A couple of these appear in Aunt Dimity Takes a Holiday. The notes' author employs the classic pasted-letters-cut-from-printed-material technique.
- The Famous Five: In Five Fall into Adventure: after George is kidnapped, the Five receive a threatening typewritten note, demanding a specific notebook belonging to Uncle Quentin, and warning them not to leave the house to warn the police.
- Mysterious poison-pen letters (together with pranks and outright vandalism) are part of a plot against Shrewsbury College, Oxford in Gaudy Night.
- Dead End Job Mysteries: In book 8, Helen starts getting anonymous threatening letters in the mail, which she figures are related to her current murder investigation. It turns out to be her ultra-religious mother trying to stop her upcoming wedding, since she doesn't believe in divorce and therefore considers Helen to be still married to her first husband.
- In The Girl from the Miracles District, Ture leaves Nikita a note serving as a Sequel Hook, promising her that "the truth" is hidden in a remote Norwegian village.
- Home: Frank Money receives a note that says, "Come fast. She be dead if you tarry." This causes Frank to start his journey home to save his sister.
- In The Hound of the Baskervilles, Sir Henry Baskerville receives a message consisting of words cut and pasted from the London Times warning him to stay away from the moor (where Baskerville Manor was).
- Parodied in Lords and Ladies, when Verence receives a technically-anonymous note which, in eccentric spelling, advises him to deal with Magrat's indecision over marriage by simply taking the decision out of her hands, gets sidetracked to point out that witches don't pay taxes and this should continue, and is signed "A Friend (Mss)". Whether this has anything to do with the witch who knows what's best for everyone, has eccentric spelling, and signs herself "Esmeralda Weatherwax (Mss)" is for the reader to determine...
- Monster of the Month Club:
- The card Rilla received announcing she'd gotten a free membership to the Monster of the Month Club. She finds out late in book 1 that it was a present from family friend Mr. Tamerow, who thought the monsters were just plush toys.
- Later on, Rilla sends her classmate Joshua Banks (whom she has a big crush on) an anonymous Valentine, saying "Let there be love". He's not as oblivious as she thinks though, figures out it's her and sends back a signed note saying "Let there be friends" in the same style.
- Mysterious notes or objects are often the catalyst for Scott Wolter's investigations on America Unearthed.
- In the Spanish series El internado: Las Cumbres León, the music teacher, receives a blank envelope. Inside there is a photocopy of a newspaper clipping with the headline "Kidnapper of girl in Lisbon remains at large"note and an attached note reading "TEN CUIDADO" (BE CAREFUL).
- In an episode of Magic City, an envelope is shoved under the door. When opened, it contains a photo with "$20,000" written on the back. The photo shows one of the heroes in bed with a mobster's wife and is clearly an attempt at blackmail.
- Murder, She Wrote: In the opening chapter of the Expanded Universe novel Rum and Razors, Jessica's been getting strange notes, all consisting of a single line - "GLOTCOYB", each time followed by varying punctuation (none on the first, a question mark on the second, a comma on the third, a period on the fourth and three exclamation points on the fifth). Shortly after finishing her latest manuscript, she sees a group of people outside her home, holding a banner with the same word. It turns out to be her friends Dr. Seth Hazlitt and Sheriff Morton Metzger, plus some other locals, who reveal "GLOTCOYB" is actually an acronym for "Good luck on the completion of your book". Jessica is both irked and relieved, since the pranks had actually been making her rather nervous, to the point where she answered the door that day with a fire poker in hand to ward off any potential threats.
- In The X-Files, Agent Mulder repeatedly receives revealing documents on various conspiracies from his informants. They try to be sneaky about it.
- The Dream Machine has it's main plot started by one Viktor finds in his new apartment.
- You can get these from couriers in The Elder Scrolls V: Skyrim. The most common and benign are letters from "a friend" who heard you Shouting somewhere and give you directions to another Word of power you can use as you develop your Dragonborn skills. More ominous is an anonymous note you receive if you start looking into the Dark Brotherhood, which consists of nothing but an image of a black hand and the words "WE KNOW." The next time you sleep after getting this note, you'll be kidnapped by one of the assassins, who makes a macabre recruitment offer.
- Silent Hill 4: Henry can receive messages from the dark world if he slides blank pieces of paper under his door. Better go check on your neighbor!
- World of Warcraft: At the start of the Mists of Pandaria legendary questline, you receive this conveniently named item, indicating that the last of the Black Dragons wishes to recruit you to save the world.