I thought they might be breeding directly on Nim. I know they're supposed to lay most of their eggs not on the snake, but I have seen quite a few of what I assume are eggs on him. I haven't seen any mites near the top of or outside the enclosure, but it's possible. I'll hit the outside of the enclosure as well when I PAM this weekend.
I scheduled the vet appointment for Monday morning, so these mites don't have long anyways.