A third dog wants to play

As I have mentioned before, this house is almost soundproof to noises outside unless they are very loud. The interior walls are also brick and the walls are thick 3 to 4 bricks thick and then they are plastered over by a near cement plaster used by builders in the mid-eighteen hundreds. So sounds do not carry well between rooms. Last night, before turning in the the evening, my wife and I spent half an hour or so playing with our two dogs. Its a nightly ritual they have come to expect. We had just finished playing with them and were preparing to turn off the lights when we heard a loud, very distinctive play growl come from somewhere in the house. It got all of our attention. The Dogs sat bolt upright and peered down the upstairs hall that runs from the master bedroom to the bathroom on the opposite side of the house. My wife exclaimed "What was that?" and our older dog jumped off the bed and ran down the hall to investigate. He came back fairly fast looking puzzled. The younger dog continued to look down the hall. Nothing more occurred and the elder dog went to his bed next to the heat register and laid down. The younger dog continued to peer down the darkened hallway and was doing so when I finally turned off the light. The sound was loud. We ALL heard it. It was a playful growl like both dogs do when we wrestle. We do not have a third dog. I have no logical explaination for it.