My best guess is that it was perhaps his mother’s or father's car, parked there for some reason – perhaps for a meeting, or class, on a home care appointment or visiting a friend - and the lifted wipers are a signal to mean he was early out of school and would be walking home, not waiting for a lift or expecting to be picked up. Some variation on that maybe?But don’t we have mobiles for that sort of message, nowadays?
Perhaps it's some well established code from which I have been sheltered, announcing that a new consignment of drugs is available or that the neighbourhood is now open for discount prostitution... Jehovah's Witnesses expected at dawn? Or possibly something more enticing... Honey produced by local bees ready to distribute to hayfever sufferers...?
OK it’s competition time - there will be points for the most credible but unlikely explanation. (And points mean prizes. What do points mean?)