I was on a group ride last year where the rider's tube had a really slow leak. The replacement tube had the same problem. We all looked at the tire carefully and didn't see anything. When the rider got home, he finally found a tiny wire embedded in the tire.

A cotton ball wiped around the inside of the tire is supposed to be good for finding these small objects -- the fibers will snag on it.

It sounds like you've used these Aircomps before. But someone new to using them might not know they leak a significant amount of air overnight, since they are thin.