As someone who used to work in the Baggage Tracing department of an airline I think I should be able to answer this one.
A bag can be what is known in the aviation world "Rushed" to a destination when it has been seperated from its owner for whatever reason. In order for the bag to be "rushed" unaccompanied a note is placed in the computer system so that the dispatcher AND the flight crew are aware. This bag will then be thouroughly screened before loading and then it will be put on the flight to ensure delivery to the owner.
NOW...If as I suspect your thread is about bags travelling on a flight which their owners missed or were offloaded then I assume that because all bags are now 100% screened in the USA for explosives and whatever else, the dispatcher and captain both decided that it was an "acceptable" risk to take the bags instead of creating a costly and frustrating delay considering they had been fully screened.
Im not sure if it does happen still in my airline but to be honest considering the current security situation I doubt it will happen again. However I personally would not be too worried leaving the states with an unattended bag, other countries however I would be very worried about it.
But this is just my sypnosis so please dont shoot the messenger!