It was in an update a week or so ago, so make sure you have the latest version. When you start the game the first time daily you'll find some of the cans have trash in them if they're close to spots where it usually appears, and the immediate area will be clear. You won't actually see anyone putting anything in them, though.

BTW, I think only the metal ones work like this - I have a square wooden one as well and it's always empty.