Post Lockerbie, it become SOP (at least on this side of the pond) that if someone checked bags but didn't board, their bags had to be removed prior to dispatch. Happened to me - TWA 747 out of JFK. Someone checked bags then didn't board. We had to hold at the gate for an hour while they searched through hundreds of bags to find and remove the offending ones - by which time we'd missed our takeoff slot. It was the start of the most unpleasant flight I ever had as SLF. When we finally got in the que for takeoff, there were dozens of aircraft in front of us waiting their turn to takeoff. Then, they closed the departure runway due to severe thunderstorms
. It was a hot summer day, the pilot shutdown the engines to save fuel, and the APU couldn't keep up. The 747 was full, so it got pretty miserable in there. We finally took off over six hours late
Perhaps something like this happened - and they left the gate without someone who had checked bags. If they did, it would probably be quicker to go back and let the passenger board than to find their bag and remove it.