When I first arrived at Wakefield Prison, Archie Hall was on a constant watch which meant staff were permanently stationed outside his cell, one night I was on the watch & about 1am I heard the sound of someone smashing a cell up, Hall was the only Prisoner on that landing, I checked the cell where the noise was coming from & there was nobody in there.
the seg unit at Wakefield, the sound of keys & doors opening & closing can be heard on nights, I have heard it as have dozens of colleagues.
when I was at Chelmsford a Hospital Officer told me about his previous establishment (Winchester if memory serves) a night patrol Auxilliary saw someone entering one of the out buildings, he shouted then got on his radio & alerted the control room, the Orderly Officer attended with a couple of staff as back up, they let themselves in & searched the building but found nothing, they left & locked up, one of the staff gave a last look through the window, he let out a scream & was visibly shaken, he then described seeing a man laid on the table with blood pumping from him.
They later discovered that during the war that building had been used as an emergency Hospital for wounded airmen.