I'm not sure what "mite off" is, so I can't speak on that or how you use it. I hope you followed the instructions though because I dont like the idea of spraying a snake directly, and then have it soak in the water its drinking. sounds wrong to me :

As far as signs, the only signs for mites are little moving spots. They can be in between the scales, in/near the yes, heat pits and mouth. They also can live under the snakes mouth, in it's err.. chin?

If you didn't see any, then it's probably just because it is very full, or even going into shed. I'm sure its fine (assuming you are 100% there are no mites)