Strange actions by one of our dogs.

A couple nights ago, at bedtime, one of my two dogs behaved very strangely. We had gone to bed at the regular time, the older dog was in his bed by the radiator and the younger one had just laid down at the foot of our bed. The younger dog, we call Shadow, perked up and began by looking down the upstairs hall. Then she started looking around the room like she was following something neither my wife nor I could see. She then jumped up and started looking one way then the other and couldn't hold still. She finally jumped off the bed and ran down the hall. The other dog was watching her and looking down the darkened hall but I'd say out of curiosity not fear. Suddenly we heard a a whine like yelp from down the hall and before I could get up to see what was wrong, Shadow made a pell mell rush back into the bedroom , made a flying leap up on the bed and proceeded to tunnel under the covers and shook so hard she made the whole bed shake. Meanwhile the older dog just sat there in his bed and watched the whole thing with nothing more than a curious look. At my wife's suggestion, I got up and walked down the hall which leads past two guestrooms to the upstairs bath. I saw nothing unusual, I heard nothing strange, I sensed nothing amiss. By the time I had gotten back to our bedroom my wife had pretty much settled Shadow down; but she didn't come out from under the covers until morning. I have no idea what the pup thought she sensed. Whatever it was didn't seem to have much, if any, effect on the older dog who simply spun around a couple of times, flopped down and went to sleep. That sort of carrying on can be a bit unnerving; but in this house we expect the unexpected. What was it? We have no idea; but as it was out of the ordinary, I have reported it.