Dog sniffing at bedroom door

     My wife had a strange occurance today. She was in the master bedroom making the bed. She said as she was making the bed she heard a dog sniffing at the bedroom door that leads to the front hall and staircase. She was discusted because that would mean the dog had either knocked down the rear stairs gate and came upstairs and knocked down the gate to the green room or else had managed to push open the Parlor door and came up the front stairs. Either case scenerio would be bad for the dog as they are blocked from the upstairs in the daytime. A snort sounded at the door which indicated the dog wanted into the bedroom. My wife said she said,"You're just going to have to wait til I finish with the bed". A minute or two later another snort sounded, at which my wife swung around and opened the door; ready to scold the offending dog. There was no dog there. She looked up and down the hall and down the front stairs, no dog. She called the names of the two dogs we have, no response. Going back toward the rear of the house she noted the Greenroom gate was still up and quite closed. Going back down the rear stairs she noted the rear stairs gate was still up and in place. Looking through the diningroom she noted the parlour door was definitely shut. The older dog was sound asleep beside the diningroom heat register. The younger one was asleep in the kitchen beside the fireplace. What made, on three occasions, first a sniffing sound and then two snorts, like a dog asking admission? Recently we have had several dog phenomena. Apparently the strange has started.