If you're looking for a hidden hole in an air mattress, the best place to start is by feeling along the seams of the mattress. If you find a spot that feels soft or deflated, that's likely where the hole is located. You can also try using a flashlight to look for any holes that might be hidden in shadows. Once you've found the hole, patch it up with some duct tape or another type of adhesive and then re-inflate the mattress.