I was also in the same office one evening, again alone and everybody had gone home (it was only a small office). I was doing some overtime because I was up to my eyes. I was working away when I suddenly heard footsteps running really fast down the stairs. I jumped up, grabbed my coat, left the computer on and all the light and legged it!! had to run past the stairs where I'd heard the footsteps. I never did overtime alone again after that !