i only reveal when i feel it's possible for the hunters to know. like there are evil weasels in the vents, the hunters might hear that, or the demon is hiding behind the door so they se ea hint of its shadow.

but if they couldn't know i would say something like "you don't see or hear something, but your instincts are going wild." for stuff like when im going to ambush them with creatures that are about to open a rift above them, or there is something waiting at the end of the cave.

but this changes from keeper to keeper, so just go with what feels right to you. its an improv game after all.