I am pretty sure snakes can't get mites from rodents, so my guess is that your snake had them when you got it (just in low enough numbers they were hard to detect).

It sounds like you are doing what is needed. Provent-a-mite is a great product. It take a little time before all the mites will be gone, but I have been using it for years and haven't seen mites since (unless they came in on a new animal). Just follow the directions on the bottle and you will be fine. You may also want to treat your other snakes as well just to be sure.