Ahhh, but of course! It's the little piece of popcorn between the white door frame and the brown door, lurking in the the shadow on the tile, just behind the door, with not enough space for B-B to get it out.
He lay there pondering the piece of popcorn for a particularly puzzling period of persistence. Then the barking started and we retrieved the popcorn piece and B-B got his coveted prize.