was working alone last week and the building was dark except my area. I went down the hall to the ladies room, and when I got back to my office the light was turned off in my section. I was fairly sure I hadn't done it, because I wasn't leaving for the night or anything. I convinced myself I must have though, and continued working.
Later on I went in the back kitchen area and saw the microwave turned on. I thought I was alone, and was a little creeped out because my boss always tells me if someone will be there late (I lock up so I have to know who else may be there). I called out "hello, is anyone here". Nothing. I jumped when the microwave beeped.
As I went back to my desk to grab my coat and keys and get the heck out of there, I turned a corner and walked right into the "intruder" which was a new IT guy who was working late on an urgent issue. Nobody told me he was there, and he thought he was alone (which is why he turned off the light in my area) and we both just started screaming.
It took us both a minute to realize how stupid it was to be so scared and he was the first to speak. "How about we never tell anyone about this?" he asked. I agreed and lol'ed on the way back to my desk.