Dog reacts violently

     This occurred last night. I was downstairs in the library, on the computer. My wife and dog had gone upstairs to the front bedroom. My wife has been fighting a cold for a few days now. This house is rather cold and drafty; so she went to bed early and was reading a book. Our dog went upstairs with her and was lying on the bed. She said the dog suddenly looked down the unlit hall and started growling. Figuring he heard me coming up the stairs or something she didn't think much about it. But his ruff stood up and the growls became vicious and he was standing in a very defensive stance. Scared her, she never saw our dog this agitated. She said he was really scared and she was afraid to touch him to settle him down. He was intent on the upstairs hall. She said the stand-off lasted a good 10 minutes or so before he slowly relaxed. Once he was back to normal she got up and they came downstairs. My wife asked if I had done anything to upset the dog. "No," I said, "I've been at the computer since you went upstairs." I heard nothing of the upstairs uproar; but thats the nature of this house. Sound does not travel well in it. Someone shouting at the foot of the stairs might be heard in the front bedroom, if the shout is loud; otherwise it isn't likely. They say animals sense things we do not. I have to wonder what my little mutt sensed or maybe I'm growing a paranoid pooch.