I actually did do this once to a particular area of advertising. I took the time to make unique codes for each one replaced, and routed the requests past my own server to record each hit. The idea was simply to find out how much they get used, as a lot of clients seem to insist on having them as the only call to action on their advertising.

In the end there were about 20 proxied URL on an extremely high traffic footpath.Either people noticed the edges of the sticker (unlikely), or absolutely nobody in my area wanted to scan any of those suckers.