#DnD #ttrpg #DMtip : Don't ask for active perception checks then say "you don't notice anything". Give PCs an in-character reason for rolling:
"You hear a sharp click. Roll perception. Failed? Ok. You can't identify it. It could have been gravel falling or a stick breaking or even giant ant mandibles clicking for all you know. You strain your ears for a few heartbeats but the noise doesn't repeat. You're not sure what direction it came from."
In-character, they know they missed something.