Mites!

You may want to treat the area around the cage as well. Mites can walk for quite a ways. Vacuuming the floor in the room daily for a short time (a week perhaps) may help.

I think PAM is a great product. However, in addition to PAM we use Reptile Relief. We take the snake out of the enclosre and lather it up with the Reptile Relief. We then PAM their tank and leave the snake in a seperate enclosure while their tank dries. Once dry, we place the snake that is hopefully mite free from the Reptile Relief treatment into the mite free enclosure. This form of attack has been extremely effective for us. I'm not saying it's the right way, but thats how we do it.