Strange Gray Dog

Perhaps I'm Hal lucinating; however, yesterday while watering plants in the eastern window of our upstairs bathroom I turned around to water the plants in the western window I spied a gray, shorthaired, terrier type dog pawing the rails on my exercise machine. The machine sits in front of the western window where there are plants growing. Using his right paw the dog pawed the closest rail to him, about three times before he vanished. Slightly larger than the playful brown and white, Jack Russell we have seen upon occassion, this dog was solid gray, had short hair and a relatively long slender tail, and short pointed ears, he seemed friendly. Looking nothing like the two shaggy schnauzers I presently own, this dog is a new one to me. My wife says she never saw a dog fitting that discription either. What ever he was, I saw him long enough to get a fairly good look. Ghost or hallucination? Who knows? But this old house seems to still be harbouring some surprises.