Strange voice

Today, I got home ahead of my wife and went upstairs to change. As I was prepaing to come back downstairs I heard the rear door open, the dogs were barking their welcome bark and I clearly heard a voice say "What are you doing?" I thought it was my wife talking to the dogs. Within seconds I rounded into the kitchen and there was no one there. I went to the door, it was closed. I looked outside, the car my wife drives was not there. In fact she did not arrive until half an hour later. Surely if a neighbor had stepped in (which would have been unusual) and asked What I had been doing they would have stayed for an answer. Also, I was into the kitchen quickly as I was well down the stairs when the question was asked. I think I would have seen anyone leaving. The dogs obviously saw something too as their barks were the welcoming type and certainly not menacing; so what they saw must have appeared friendly to them. But where did the speaker go? Strange.