Side Note, Full Story

When a notion mentioned in passing in one work becomes the full story of another work by the same author. Most often a book trope.

Examples of Side Note, Full Story include:

Comic Books

Nevada - Courtesy The Other Wiki [1]: "The origin of the character is to be found in a Howard the Duck story that contained a "mandatory fight scene" between a Las Vegas chorus girl, an ostrich and a standing lamp. Neil Gaiman said he'd like to see that story. So when Gerber was asked to come with something original by Vertigo editor Karen Berger, he created Nevada."

Terry Pratchett - The Amazing Maurice and his Educated Rodents (started out as a passing reference in Reaper Man)
The Star Wars expanded universe does this kind of all the time. Not only is a backstory provided for every character who appeared onscreen in the movies (and even some of the Faceless Goons), but you also get stories that explain exactly what a "nerf herder" is.
China Mieville's Bas-Lag Cycle is full of these. Among a great many other things, the protagonist of The Scar is mentioned in one line of Perdido Street Station as being Isaac's last girlfriend.
Sauron was briefly alluded to in The Hobbit.

Douglas Adams - Starship Titanic (started out as a passing reference in one of the The Hitchhikers Guide to The Galaxy books)
The Legend of Zelda: Ocarina of Time is an expansion of the prologue detailed in A Link to the Past's intro cutscene. There is one deviation from the prologue, though: in Ocarina of Time, Link lives.