Adfreak tells us that "Chicagoans feast on bus-shelter ad snacks". Seems that someone at Ogilvy had the bright idea to put real snack bags inside a poster site, with the goal to remove the snacks little by little to reveal the line “Too much good stuff.”
Looks like some hungry people didn't care to wait for the reveal and went straight for the good stuff (but oddly, left all the Doritos - are they that bad?). The work of vandals or hungry homeless? Your guess is as good as mine.