Hi there,
I've expereienced this arriving at LHR on a couple of UA flights. As mentioned already, it seems as if they're looking for something rather specific.

The time I've been most freaked out was arriving in LHR T4. My bag had been damaged and getting that sorted took about 10-15 minutes. So I came out by myself to be confronted by a barrage of newspaper photoographers and TV camera crews all photographing and filming me! Now that's a much bigger shock than customs officers I can assure you.
Anyway, after about 5 seconds (it seemed MUCH longer) someone shouted "No - not him" and all the lights went off. I was rather intrigued about the fuss so I asked one of the camera crew what was going on. Apparantely, some stupid English girlie had been caught smuggling drugs out of India and was being deported back to England after a while. She was due to arrive about when I did and the cameras were all a little trigger happy!

Jason
(Who leads a nice semi-anonymous life)