3 IRA men went up the stairs, and the leading man turned a corned an saw (in a mirror) a man with a gun in his hand moving towards him. He fired, believing that he was about to be attacked. The mirror shattered, and the sound of gunshots apparently enabled their vistim to get away from his room (bit unlikely this) so that by the time they got to his room it was empty. They picked up some papers they found in the room and left via the front door.
Little more can be found about this. Like most failures on Bloody Sunday, nothing is written, nor the IRA men identified. Gleeson's book Bloody Sunday merely calls this a "private source"
Addresses of Raids