I would have chalked this up to imagination and to eyes playing tricks on me, except that the dog saw it too – quite clearly.
I was at the house of a friend, Pat, and he had excused himself to go the bathroom. His dog, a bouncy yellow Lab named Jake, and I were playing tug-of-war in the living room while we waited for Pat to return. I heard a rustle behind me, and I saw the dog turn his attention to something behind me. I assumed it was Pat, especially when Jake’s tail started wagging and he abandoned our game to walk over there. When I turned around, I saw Jake approaching Pat, who was walking from the bathroom into the little library off the living room. Jake was walking behind him and wagging his whole body the way Labs do, but then, as Pat went into the library, Jake stopped dead and tilted his head to the side, then looked at me, then back into the library. He took a step toward the library door, then danced backward a little bit and tilted his head again.
By then I had reached the door to the library. I looked in, and no one was there. I went in, checking behind the door, but no one was there. The window was blocked by an enormous chair and stacks of books which had not moved. Where could he have gone? Then I heard the bathroom door open, heard Pat walk out into the living room. He stood in the door of the library, staring at us while we stared at him. I told him that he had already walked in there. He assured me that he had not.
But Jake and I know better.