I'll probably get shot down in flames here but it's not a great idea to tell passengers if theres a lot of bags being left behind. The crew would get endless grief for however long the flight is when ultimately they're there for safety and wouldn't be able to do anything about it. There would always be someone ready to start a mutiny and go OTT which is not good in an aircraft scenario. It would also impede on service delivery......whether that be BOB or complimentary. Chances are most would travel without their luggage anyway so inform them on arrival.
Baggage shouldn't be left in an ideal world but there are obviously occasions where it is necassary. Earlier this Summer LGW had baggage belt issues and for my airline if they waited for the bags that were missing the schedule would have ben a right off. Despite all the understandable issues surrounding left bags, most pax would rather the schedule keep on track.
However, an airline should always know where large amounts of left bags are.