It's a British tradition, innit. Get a drive-thru McDonald's, sit in the car eating it, then wind down the window and chuck it all out before driving off.
I walked passed someone's parked car once, and this box came out the window. Being a helpful and not observant sort, I picked it up, thinking it was something they had accidentally dropped. When I realised it was litter, I had a tricky choice. Did I (1) put it back through their window, risking getting my head kicked in, (2) drop it again, thereby creating litter, (3) walk off with their litter and put it in a bin, looking like some sort of servile litter slave for morons. After some thought, I did 3. No easy solution.