Go on then. How the blinking flip did you lose a staircase?
I have no wish to hear about the you, in the shed, in a dressing gown, with the Water Board bloke. Please let me retain *some* innocence...
While I was a Volunteer at the Pump House, someone from the local council visited. He was looking for some stairs or a stair case. I pointed him in the direction of the staircase, but he returned seconds later, saying that was not the one he was looking for. I have been going there for a year now, and explained that I thought that was the only ones we had. He was quite sure that we had some more stairs or staircases and so we proceeded to look for them. At that time, my "boss" was taking a break and walking her dog in the park next to building, so I told him to look out for someone walking a black terrier. He could not find her, so we resumed the search. We guessed that any sensible stair case would start on the ground floor, so that is where we started our thorough search.
It turned out, when my boss returned, she explained that the staircase was under the tables where the visitors computers are.

Apparently, because it is under the tables, it is easy to overlook.
The stairs lead down to the basement.