I had a similar experience last night.
It was a dream involving a mirror. I wasn’t interacting or looking at it as I was busy with something else in the dream (I cannot recall). Suddenly I looked at the mirror with trill, there was no reflection of me. I immediately knew what to do with the mirror, to make it show something scary. I looked at it and push it to show something. After a while, out of nowhere my face with my palms touching the other side of the glass appeared in the mirror with a scary look, I also heard a scream coming from behind then I woke up.
I need to mention that it was just a normal dream. The thing I am concerned is that my subconscious is somehow programmed to do this thing, every time I see a mirror in dreams. The problem is I don’t know how did I managed to do this, although I can recall all the mirror events I still can’t reach to a clear conclusion.