Have not yet had this problem, but I think mites cycle through different life stages, and if they have fed, their coloring becomes darker, or their age reflects their coloring.

I just googled 'snake mites' a few minutes ago and found a lot of information and also, how to treat them.

Apparently, from what I read, mites happen. The best thing to do is educate yourself on what to do if they do show up and have those materials on hand. Good luck