Ghostly dog again

Yesterday I worked at the computer which is in the Library. The Library can be entered by two different doors. One of the doors is almost beside the computer table and leads from the dining room the other doorway is directly to my right as I sit at the computer and leads into the Den. I was reading mail on my computer and scratching the ears of my grey schnauzer when I saw the movement of a grey schnauzer from the Diningroom through the Library and into the Den. It didn't register with me at that time that anything was amiss and my older grey schnauzer was enjoying getting his ears rubbed and did not react to the presence of the "other" dog. Moments later I caught the movement of a grey dog going from the Den to the Diningroom and that was when it hit me that I have only one grey dog and I was scratching his ears. Our other schnauzer is a younger coal black dog, easily noticeably different from her older grey companion. I have had several episodes of dogs appearing where they couldn't. But my real dogs do not seem to pay any attention to them. I saw this dog trot from the diningroom to the den and later come out of the den and go to the diningroom. I was not looking directly at him, I have excellent periphial vision, but this was a bit better than "out of the corner of my eye". This is the type of thing that I seek an explaination for. The first thing that goes through your head is "ghost"; but I'm far from sold on the idea of wandering spirits either human or animal.