This gun CAN FIND SECRET ROOMS!
Contrary to what it might say on the ammonomicon it is able to find where secret rooms are.
The clue you might even consider to be in the book itself.. "sticky".
When fired towards a secret room, darts will NOT "stick" to the wall, but rather fall down, as they would if you hit some torch!
The Dart Gun is a gun that fires darts. The Cultist starts with this gun.