I believe in science over ghosts so if that's all it did id simply chalk it up to seeing things and leave it at that, if it happened more then once then I'd swap to believing I was crazy while still ignoring it and keep doing that until someone else could confirm that it was actually a thing not just me. Its one of those things, if I was in a reality where ghosts were a thing , I would have options and steps to take and would be more accepting as society in general would be accepting but as it isn't and Im not I have little to no problem with this, I saw "someone" enter my room many many years ago that wasn't there but I was like 7 and stayed up to like 1am which was way past my bedtime so I don't really think there's anything in it and id treat this the same.