I have posted this account a number of years ago but I can't find it and I'd like to share it with you.
The occurrence was about 30 years ago when, as a student, I worked as a security guard in London. Although I've had quite a few strange experiences this was by far the most interesting.
It was summer on a sunny Saturday morning. By this time I'd been promoted to area inspector so I had a number of teams to look after across London. Over the radio I'd been speaking with a friend and colleague; it seemed a good idea to meet up for a coffee and one of my sites due a visit was The South London Women's Hospital in Tooting. We agreed to meet up there. This hospital had been closed for a number of years, had suffered squatters resulting in a team of three stationed security guards and a visiting dog unit to assist in covering what was quite a large area.
Like most hospitals, there was the original smaller building, added to by large corridors and added structures.
I'll describe the layout as it is relevant and saves further explanation.
Through the large, wooden double entrance doors there was a further set of double doors taking you into the short hallway with a reception room on the left. To the right were doors to some wards, more or less straight on led to the back and new parts of the building with access to the gardens.
Left was a corridor, entrance door to the reception room, huge lift and two day rooms or waiting rooms, one at the end of the corridor. To the right were windows into an over grown quadrangle. The area was light and spacious. At the end of the corridor there were two glazed double doors leading to an oak staircase that only reached a first floor. This staircase is a noted design feature and of historical interest.
It accesses a short section comprising a very short corridor with a dead end of fire exit doors. On both sides there were two small rooms, well lit, and they were probably private rooms. They were empty.
'S' and I met at the front with our small vans squeezed in to the small drop off point. Bounding in to the place we stuck our heads in to the reception room where the guards were and volunteered to do a patrol for them if they put the kettle on. All was well, on went said kettle.
As we were about the start the patrol we both saw a lady walk out of the far room on the left nearest the double doors towards that fine staircase. We launched in to a run. Both us calling out: "Excuse me madam. You are not permitted here." (I know, how polite we were...). We repeated this whilst saying to each other that we had an intruder.
The figure kept walking away from us and through the double glass doors which we got to and through as the figure was now up the staircase and I got a side/front profile for a moment. The figure disappeared, vanished.
We were at the top of the staircase. I said to 'S' I'd stand point as the staircase top was the only exit point and he was much taller than I was! He returned within a few minutes saying there was no one there. I asked him to stand point and I'd look. Nothing. So we shrugged our shoulders and went through the rooms together, checking the security of the windows and noting the fire door, locked and further secured by padlock and chain.
Our conclusion was that we had seen an intruder but they were no longer on the premises. I personally wrote up a description in the duty log book as we supped on our coffee.
It was only some years later that on reflection, I realized, knew for certain that I, we, had seen a ghost. This is where critics and psychologists get it completely wrong: our minds made the event look completely normal. The fact that the figure vanished before our eyes was washed away by an assumption that the figure had kept going up the stairs and we had lost sight - impossible in fact.
So the detail: the figure was female, dressed in blue. She was holding a file or clipboard or similar to her left arm against her body. We heard her footsteps on the wooden floor. I believe she opened/pushed one of the double doors to go through it but I can't be certain of that. Indeed my mind, rationalizing as our minds do, saw it as so because a person can't walk though a shut door. I clearly recall not being able to make out the facial features at all and I should have been able to do this. There was no other exit and we had it covered. A ghost? Yes of course. A nurse.
The hospital is now a Tesco store. I believe they were unable to save many fine architectural features as English Heritage refused to list them. Of personal interest, I have met quite a few people who only later realized they had a ghostly encounter; some incidents are more obvious than others I suppose.