I was out last night for a beer with a few coppers and told them OP's story (with details changed). Their consensus view:
* OP flagged himself up for suspicious behaviour when he blagged his way into the first building
* Receptionist then no doubt put his details out over the local "keep an eye out for" net - most pubs, clubs, shops and hotels in bigger towns and cities are now signed up to one of these
* There was a plainclothes patrol in the area and a sharp-eyed PC/DC spotted someone matching OP's description
* They detained him and asked for his details and he started trying to tell them the law
* He was either (a) a big, dangerous-looking fucker or (b) a little guy with a big mouth ... so they secured him
* They talked to him a bit more, got what they needed and concluded he was (a) not carrying semtex (b) no threat (c) a stupid little cunt
* They let him go
A perfectly routine night in any big city...
I find the OPs post plausible, and the above post plausible.
It reminds me of an incident long ago. I was walking away from my car that I had just parked in the city centre. Two plain clothes cops holding up ID walked swiftly towards me. They asked me the reg no of the car that I had just left, which I then told them. I cant really remember but they probably said that a lot of cars were being broken into or stolen recently in the area.
I think that the OP aroused suspicion by walking in and out the first building looking a bit lost, and the receptionist acted on it. There was possibly a lot of thefts/break ins in the building prior to this. Maybe the OP is a youngish person and they tried to bullshit him with ''illegal'' prossy activities nonsense, just to make him feel more uneasy? Maybe they were just being cunts, who knows.
He need have no worries about anything on his ''record'' as he's not done anything wrong. Sometimes, one can have unpleasant encounters with cops, even though you have done nothing wrong. Some can be pleasant enough and some can be pigs.