mystery 
cloak and dagger, closed book, conundrum, enigma, problem, puzzle, question, riddle, secrecy, secret, teaser


I would prefer conundrum first because it sounds strange, and a bit rare as "schmilblick" in french.
(French n. énigme, problème)

The real meaning of conundrum is a puzzle or a riddle designed to test for lateral thinking.
But I think you are inviting people to solve a riddle so conundrum could be quite good.