I worked for the NHS as an engineer, one day, I was carrying out inspections in a derelict tunnel which although unused had steam services running through it. This tunnel was approximately 80 meters long, and you could see from top too bottom. In the centre was an old fire door, I was carrying out an inspection when I heard some activity.
I heard a creaking sound, and as I turned towards the door I saw that it was opening slowly. As I watched it seemed to stay open for ages allowing me to view the bottom of the tunnel, I was the only one there. The door opened flat completely open to the wall, and then suddenly slammed shut! Keep in mind this was a pretty heavy fire door.
As I though someone was playing a joke on me, I ran to the door and opened it no one was there, as the door opened flat too the wall, no one could have been behind it, no one was there but me. That was enough for me. It spooked me so much I left right after. I found out later, in the past, others had witnessed things like this also.