Dog in a hurry

Here we go again. I was writing at the computer, today around 11:15 edt, when I heard a thump behind me that sounded like a dog jumping off a piece of furniture and the dog ran rapidly out the Library door. I was busy with the computer so I only saw the dog in my periphral vision. I thought to myself just what on earth excited the pooch so much. Normally they only react by jumping and running when they hear the doorbell, or the mail truck or my wife coming in. There had been no doorbell, it was way too early for my wife and both dogs spring into action with the sound of the mail truck, not just one. So I turned around to see which dog left in such a hurry. They were both sound asleep, one on the chair by the phone and the other on the rug in front of the chair. But somewhere I had seen an extra dog. Or thought I had; or something and what made the "thump" that alertyed me to begin with. I seem to recollect that the dog that ran out the library door was brownish. At least I think it was brownish. Neither of my present dogs is brownish, not even close, one is lightgrey and white and the other is so black a piece of charcoal would leave a light mark on her. These episodes occur so fast and you never expect it; it leaves you scratching your head and wondering what on earth happened?