If they are treating for mites at the store then it looks like you have mites. And I doubt very much that they are all dead. Usually a mite will burrow between scales and it won't move at all. If you wet a paper towel with Reptile Spray and wipe over the area they will usually come crawling out within a half hour. I had a snake with spots all over that looked similar. After wiping down and checking back in 30 minutes there were mites crawling all over the snake, it was pretty creepy.