Sometimes we can have a random hallucination, and it never comes back. I've had it happen once (dont' want to describe it here though). It scared me so much I was looking over my shoulder for months... even years later I'd think of it and start looking round. But in twenty four years I've never seen it again. This could just be random, not a sign of anything further wrong. You're right to see your pdoc, but don't be too frightened. At least you worked out it wasn't real.