Dog in the mirror

Yesterday evening as I was finishing up shaving I distinctly saw a little black dog, in the bathroom mirror, enter the room and bounce over toward me. I turned around to scold what I thought was my little black schnauzer, for coming around or over the downstairs gate. But there was no dog there. I looked all around, certain she was somewhere in the bathroom. She wasn't anywhere upstairs because a minute or two later I heard my wife call both dogs and I heard the gate open and the sound of two stampeding schnauzers coming up the stairs. So what did I see? This was not a brief impression, what I saw must have been several seconds in duration. I was certain enough that I turned to chastise the offending dog. The bathroom is large, as bathrooms go and it is well lit. I definitely saw something. If not my younger black mutt, then what?