Disappearing dog

Last night, after taking the dogs out, we all went upstairs to bed. It is still 80 degrees at bedtime and humid to boot. Ordinarily we open a couple doors downstairs so that when we turn on the exhaust fan it can pull from downstairs as well as upstairs. But it was so uncomfortably hot last night that I closed the heavy hall door so the fan would exert all its energy in pulling air through the upstairs. In order to maximize the fans pulling power (Its located in a window in the upstairs bath which is at the opposite end of the hallway from the master bedroom),I also closed all doors to the side bedrooms and western end of the hall. Everything is closed tight; the fan is pulling air only across the master bedroom and down the hall to the upstairs bath window and out. The dogs have both come up with us and even though it was stuffy I still played with them a few minutes before we turned in. I had to get up once during the night for a bathroom run and at that time the older schnauzer followed me down the hall and back to the bedroom. Then this morning when we got up my wife says "Wheres Rowdy?" I look over at his bed and its empty, he's not in the bedroom so I figure he has gone down the hall to the bath for some reason. But he isn't there. He is nowhere upstairs. So I figure he must have forced one of the bedroom doors...nope. The doors are all tightly shut. By this point the younger dog is insistant on going downstairs so I throw on some clothes and open the hall door and standing at the foot of the stairs is the missing Rowdy. How did he get downstairs? I have absolutely no explaination; yet this is the second time he has mysteriously gone downstairs when there is no obvious way he could have accomplished it. I wish dogs could talk.