I don't know how long rat mites can live between feedings. Unfortunately a snake mite can go months between feedings so if you saw snake mites 3 weeks ago then some could easily still be alive.

If you don't want to use PAM or other permethrin-based insecticides then you could always use Reptile Spray on the snake and hot water on the enclosure. Anything over 150*F will kill mites and eggs. I knocked back an infestation pretty quickly by using a heat gun - the kind for stripping paint or wallpaper - set on the lowest setting of 150*F. A steam cleaner would also work.

Full disclosure though - I also used permethrin on the rack and tubs, and a No-Pest strip in the room for one day a week, repeated for 8 weeks. I really did NOT want the mites to come back!