This is the latest craze with todays....ahem...."urban" youth, largely made popular by said youths posting similar events conducted around the nation on sites like youtube and other "urban" specific sites. There is video of this sort from Detroit, D.C., and other locales across the nation.It might of been a "senior prank" gone wild.
As a customer, I would simply leave.
As a clerk, I would call the cops.
As an owner, I would exercise better access control after dark.