Whistle while you work

I watched a lad defacing posters in the underground this morning.  Just plastering stickers over faces, that sort of thing.  But, and this was the thing that really struck me, as he left the scene of each petty crime, he actually put on a cartoon-style whistle and an exaggerated carefree I’m-entirely-innocent amble.  And it looked to all the world as if he believed this made him completely inconspicuous.