I deplaned from a BA flight in Chicago this spring and ground staff had cello-taped printouts of 40 or 50 names to the jetway wall and recited loudly, over and over, "If your name is on this wall, your bags are still in London!"

IME they do not announce to the cabin that some or all bags did not make it aboard so as not to upset people in transit. I guess the strategy is to upset them immediately upon arrival. In any case that flight, like OP's, was on stand at T5 for a long time before departure, no apparent chaos going on, and we left about :30 late. No apparent reason for the fail.