This happened to me when I was eighteen. I'm now in my forties but remember it like it was yesterday. Before I start I would like to mention that I don't scare easy. I tell you this for a reason. I was once a firefighter and had faced death many times. I also did some sky diving and on my 2nd jump my chute didn't open and I had to quickly go to my reserve. These and other times were fearful but never was I as scared as the time I'm about to tell you about.
It was about 10:30 am one weekday and I had laid down on the couch in the fire station I was at and dozed off. About an hour later I woke up and rolled over on my back and there at the end of the couch was an old man standing between the door and I and glaring at me. It was apparent he was angry and as I lay there trying to figure out who he was and why he was mad he spoke. The instant he spoke I knew this was not a person. Every hair on the back of my neck stood up and I sensed evil. In a non human garbled voice he said "what in the hell do you think you're doing?!" Again, I've never been so scared, but my escape was blocked and my only thought was, I'm not going down without a fight. I said "what the f***k does it look like I'm doing? F**k you I'll spit in your face" and I started to get up and go towards it. Then I just passed out. I don't know if I passed out from fear or "it" caused me to pass out. I just know I woke up a little while later. I had a headache, I think from hitting my head on the arm of the couch. I quickly looked and he was gone and I got out of there.
To this day I don't know who he was or what his problem was. I never had any other encounters but other firemen had bad experiences with something in that building.