I had my house alarm go off at 3:30AM a few years back. Scared the crap out of the wife and me. I got dressed in about .6 seconds and headed towards the stairs. When I got there, I found the front door WIDE open and the cats all crouched down on their haunches looking into the foyer with their hair all up.
They were very spooked from the alarm going off.
I closed and locked the door and then checked the entire house 3 times. I think, to this day, that the door wasn't fully closed and the deadbolt hadn't engaged the slot in the frame. It was a windy night and the wind must have blown the door open.
My wife and I never did go back to sleep that night.