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.

Anime & Manga

 * In Oresama 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.

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 hollywood 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).

Live Action TV

 * In The X Files, Mulder repeatedly receives revealing documents this way from his informants.
 * 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.

Video Games

 * 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!