Pay it forward

My apartment is quite unique in it's trash collection.  You know you live in a city's premier apartment community when they provide valet trash service.  Yup, that's right.  You leave your trash at the curb and it magically disappears for you.  BUT they only pick it up on MWF before 9am.  So on Sunday night I left a bag of trash outside of my door to be taken downstairs when I left for work Monday morning.  When I emerged this morning someone nice had already taken it down for me!  And it was filled with glass jars, a watermelon, three year old TV dinners that were blocks of melted ice . . . in other words it was not the world's nicest bag of trash.  I assume its' the people across the hall from me (the ones that slam their door so hard that they rattle the pictures on my walls every time they enter and exit their apartment) but now I have to figure out how to pay it forward.  Ideas?