Mysterious Note

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
  • Invitation
  • etc

Not to be confused with Brown Note or any other sound related trope, although it can contain, or be a recording of any kind.

Examples:

Anime & Manga
  • 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.

Fanfiction

Film
  • 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."
  • The Incredibles. The "note" is in the form of a digital tablet with a video message inviting Mr. Incredible to do secret superhero work.
  • In Unsane, a horror/thriller author receives one about a series of murders related to his latest book.
  • Memento: The hero receives a few of these. Some of them from himself, since he suffers from anterograde amnesia.

Literature
  • 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).
  • Mysterious poison-pen letters (together with pranks and outright vandalism) are part of a plot against Shrewsbury College, Oxford in Gaudy Night.
  • A couple of these appear in Aunt Dimity Takes a Holiday. The notes' author employs the classic pasted-letters-cut-from-printed-material technique.

Live-Action TV
  • In The X-Files, Agent Mulder repeatedly receives revealing documents on various conspiracies from his informants. They try to be sneaky about it.
  • 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.
  • Mysterious notes or objects are often the catalyst for Scott Wolter's investigations on America Unearthed.

Pinball
  • In WHO dunnit, Hardboiled Detective Nick Spade receives an unsigned note asking him to meet someone in the basement of Tony's Palace. It's actually a setup for an ambush by the murderer.

Video Games

Webcomics
  • In Autumn Bay, Intrepid Reporter Felicia Kingsley receives one from "a friend" when she is supposed to meet a contact. It is pinned to a post and has her name on it.