I was pulled up for taking some video shots of an aircraft on the apron from inside the departure area (airside) of a UK regional airport two years before September 2001.
An airport security officer and a plain-clothed police officer (from the local Special Branch stationed at the airport) approached me and told me what I was doing was not permitted - there weren't any prohibition notices exhibited.
We had a civilised conversation and I played back to them what I had taken (at my suggestion, not theirs).
I also pointed out that I could have obtained the same shots from the public road alongside the apron, albeit from a different angle and from further away though I had a 20x optical zoom even in those days.
They agreed this was the case but said they were concerned that anyone filming inside that part of the departure area could also take shots of the security area.
There was no point in my pursuing the matter further - we parted on good terms.
I think that's an important point - never argue or lose your temper in such circumstances as there will be only one winner, at least on the day.