Better safe than sorry.
Often, it seems there was no threat to begin with but they spotted something and are being cautious.
The wife and just returned from a trip to France and England. While going through Heathrow on our departure a small dog came running up to my suitcase, sniffed awhile, and then stopped. An airport security person told me there was something in my suitcase.
She then asked me what was in there other than clothes. My wife said some paper money was inside.
It seems much paper money has drug residue on it.
The security people seemed satisfied with my wife's answer and sent us on our way.
But for a while I thought we'd be hauled away to some back room and given an anal inspection or something.
