@phillipa: "I think you are missing the point. Their bags were already checked and on board but removed by MH before the aircraft departed. So it is highly relevant to ask who were they? Did they ever show to collect the bags? What was in the bags?"
Not so, I believe. I believe they were checked in but it wasn't confirmed the bags were loaded. They could have been connecting passengers from another flight booked through and boarding passes issued, but they never made it to the gate. Various possibilities, including late arrival of their inbound connecting flight, would preclude them from boarding and also preclude their bags from being loaded.