Like something out of 'Allo Allo' (for those who are familiar with the English sit-com taking place in occupied France during WW2) I was reading a book about the Nazi occupation of Poland. It describes an anecdote about how a Polish courier who was wearing a German uniform was discovered when crossing to a different area. The Germans told the Pole to strip to his underwear. They then noticed that he did not have a military stamp on his shorts, which showed he was not a German soldier. He was promptly arrested and sent to prison. From the prison he smuggled a message to the Polish resistance telling them that in future all couriers should have properly stamped underwear. The Germans thought they had discovered a foolproof way of detecting agents. They forced everyone, including real German soldiers, to drop their trousers for underwear checks. The Germans were fuming over the indignity of having to go through embarrassing checks.