Recycling Center?

When the local characters manage to sneak their booze into the library what compells them to leave the empty bottles behind?  It's not like they leave them stashed under tables or in the trash cans. They instead stash them behind books, on top of shelves or in any crack or crevice they find in the library.  Do they think their benefitting us by allowing us to collect the recycling fees as some sort of library 'donation' or are they too drunk to find the trash can?