I clean up other pax trash quite often --- this usually happens when I get to my seat and a traveler before me with different views as to whether or not the seat-back is a garbage bin has left their trash in the pocket. hand it off to FA instead of riding with trash for hours. Would never even think of leaving it for the next pax or the cleaning crews.

ps - IMO, leaving trash in the seat-back is bogus to do even to the aircraft cleaners who are human too (robots not yet in place) -- if pax don't want to throw their own trash away the least they probably should at least leave it in plain sight so cleaners don't have to play 'go fish' with rubbish of unknown origin or character.