Thanks a lot for your appreciation !
Yes, we absolutely should have put a tutorial in there somewhere, it's definitely the biggest weakness of the game.
But now dumb dev story time !
The guards' AI is actually really simple : when a guard (or their camera) sees you, they get an exclamation mark and follows you. The question mark was just feedback for when invincible dark guards get shot, as a way to show the player they couldn't be killed. However this kinda backfired with the guard walking is the laser room (top left) because his exclamation mark kept getting triggered by said lasers.
This is probably what you were thinking about !