I have had this before too! Once on the way back from NAS on the 767 in 4J/K arrive at the seat for a night flight to find there is no pillow on my seat, just a blanket, I let the cabin crew know (who said there was definitely a pillow on each seat when they boarded) and after 10 minutes they came over with one and apologies that it was so crumpled and that they didn't have an iron on board! We laughed it off and I went to sleep. On disembarking at LHR i spotted 2 pillows on MR 4F's seat.... very naughty.

On the other hand I have been known to take extra pillows (and the odd extra blanket to use as a mattress topper) from neighboring seats once the plane has taken off, the seatbelt signs are off and its definitely clear no one is occupying the seat.