Noisy Night

Last night at about 2 am one of the smoke alarms started going off.  Bob got up and looked around the house and there was no fire or smoke, but he did not know what alarm went off.  Then about an hour later it went off again, but still not knowing which alarm it was, Bob went back to sleep.  Then 15 minutes later it went off again, so Bob went in the hall and waited for it to go off again.  It turned out being the one over his desk in the computer room, so he pulled out it's battery so he could get back to sleep.