Mites Love the face, and that is one of the easiest places to spot them. Just because you only saw one there doesn't mean there are not more. I think it is a safe assumption that you have them, and I would start treatment.
If you are still not willing to leap into treatment just yet, observe your snake... is it soaking more? Is it more irritable? Is it less active? These are signs that your snake is uncomfortable. Mites are like snake fleas... they are uncomfortable to have.