I know the word exists, as I've used it before. It's essentially an element added to a book, piece of art, or even software that serves as a nod to information shared between two people. It's an element that would only be understood by the author and a specific member of the audience. I'm also fairly certain that this specific word was used almost exclusively with literary works.
As an example, when programming I'll often add an "easter egg" to my code. Sometimes it's a variable named after a girlfriend's dog, or a comment containing an inside joke, or even an anagram that only a certain sly colleague will find.
The word is not:
- "inside joke" - It's not always a joke.
- "easter egg" - It's not always meant to be found.
- "nod" - It's a bit more personal.
- "hidden message" - It's specifically related to literature, and I'm pretty sure it was one word.
I've been going crazy trying to remember. I'll look for some examples.