You make a valid point about unaccompanied baggage so I hope this explanation helps to understand how the system acually works.

The bags you describe above are not loaded willy nilly onto an aircraft to merrily fly their way around the world unticketed and undocumented. When a bag does not fly on the same flight as it's owner as series of safety measures come into play. If for example you are on a connecting flight but your bag tag numbers have not migrated across to the connecting flights computer system then baggage handling will put a large red tag on the bag saying " Refer to Flight Dispatcher". The bag will not be loaded into a baggage bin but sent out loose to the aircraft. The Dispatcher will check with the gate staff that the tag number is in the system, the passenger will be paged to show appropriate baggage tags to the staff and add them into the system if required. Failure to do so means your bag will not travel. When this happens the bag will be sent to a special screening unit where the bag will be scanned to ensure it does not contain anything untoward and will be manually searched if it does. This additional screening is paid for on a bag by bag basis by the airline in question and is not cheap. Having cleared security the tag number will be entered on a special list made up for each flight known as a "Rush Bag List" which is double checked (each bag and tag number) by the Dispatcher and/or head loader before loading onto the aircraft. Any bag not listed or incorrectly listed Does Not Fly..