It could still be mites that you just haven't seen yet (they can be underneath the scales sometimes and can be hard to see if it's a mild infestation), or he could just be soaking for the sake of soaking. Personally that's why I prefer to just treat all my new snakes for mites right away when I get them, just in case. It could just be nothing (and likely is just nothing) but if I were you I wouldn't want to wait and see.