[quote] But it can't be that common for the aircraft to depart with all the bags missing can it? <hr></blockquote>
That is my point exactly, it is not something which happens often which suggests there is good reason for the bags being left behind.
It is probably quite right that the crew on-board the aircraft do NOT tell the pax that bags are missing, they are in a confined space with the pax and have no idea what is going to be done to resolve the issue. It is the job of the handling agent to reunite luggage with passengers.
Just spare a thought for the poor sole at the destination airport who needs to take a barrage of abuse from an entire flight load of passengers - all of whom have onward connections/business meetings etc and demand that the bag is delivered, NOW!!!