One time I DIDN'T get stopped....
Once, many moons ago, I was working in Belfast, during the late seventies. So, quite an, er, interesting place to work at the time...
I was standing in the queue at Heathrow, waiting for the daily shuttle to Belfast, when this sharply dressed man flashed some sort of ID at me and asked me whether I was going to Belfast for business or pleasure. I told him "Business", and he said "What sort of business?" I said "I'm installing a computer system for the RUC". That was the end of the conversation, and I was allowed back into the queue.
(RUC - Royal Ulster Constabulary)