The Purloined Letter

Wikipedia contains spoilers

Plot

Someone is openly, arroganting blackmailing a royal by threatening to reveal a compromising letter. The success of the scheme rests on being able to produce the letter on a moment's notice. Despite being waylaid several times, and having his apartment ransacked, the blackmailer remains serenely confident. French police even dismantle his furniture and walls with scientific precision, yet are unable to discover the document.

It turns out to be hidden in plain sight, in the 19th century equivalent of a modern in-box!

This story in particular aroused much speculation, especially between the French psychologist Jacques Lacan and philosopher Jacques Derrida, who exchanged countless letters based on this story on the nature of desire.