This is kindof hard to figure out. I assume you are the first level. Go to the right of the house (before entering it). You should find the fear scope. Whenever you think you found a trap, you go into first person mode, and select the item with the scope. It takes ten points of adren. to mark something, so make sure you get it right.
The scope will not stop fights, so don't bother tagging the battle triggers.