Shoes flung over a Telephone line

I see this phenomenon quite a bit around Dublin and have been told it advertises the sale of Drugs nearby, certainly it seems to occur in the more deprived areas of the city.
If it is the case and these sellers advertise their whereabouts you'd wonder why the police don't have more success apprehending them?
Is this an old wives tale or truth? I mentioned it to a customer tonight in the Cab and she laughed me out of it, saying it couldn't be true!