With positive passenger/bag matching rules, what happens to a bag that does not arrive with the passenger? According to one answer to Why offload bags for missing passengers when flights are allowed to carry unaccompanied non-PAX air-cargo in holds?, the requirement for Positive Passenger/Bag Matching was introduced in 1988 following Pan Am 103. According to another answer to the same question, airlines cannot accept cargo from the general public.

Yet on a number of occasions when I've flown commercially as a passenger (including once to the US in the summer of 2001), for one reason or another my/our bag(s) have ended up somewhere else, or at the very least, not showed up on the conveyor belt at the destination airport along with the other passengers' bags.

What usually happens at that point is a conversation with an airline representative, and possibly one's card issuer or other travel insurance provider; and the bag arrives a day or two later. For the passenger, that's an inconvenience, but thankfully not usually a disaster.

However, what happens to the bag in the interim in such a situation?

Clearly, the bag very likely wasn't on the same plane as the passenger. This could mean one of two things:


The bag was not loaded onto any airplane (so it's still at the origin airport).
The bag was loaded onto some other airplane (so it's somewhere else).


The case of the bag not being loaded onto an airplane is strange, but doesn't seem to violate the requirements of PPBM; it's just sitting in a cargo holding area somewhere. If the bag was actually loaded onto some other airplane, that would violate PPBM, since there's no passenger on that flight accompanying the bag. (That said, I don't know if it would be possible to exploit this as a security weakness. Probably not, at least not reliably.)

Once the bag is found, it needs to get to where the passenger is now. I can see two basic ways to do that:


The bag is moved by ground vehicle (car, truck, boat, ...).
The bag is moved by airplane.


Driving a potentially long distance potentially just to get a single bag from one location to another seems the less reasonable alternative (it could probably easily cost a significant fraction of the profit from that whole flight), but on the other hand, if the bag is loaded onto an airplane, it definitely is a bag travelling without an accompanying passenger and in this case, it's known as such to the airline, thereby seemingly violating the requirements of PPBM.

If it were to be shipped as cargo, that would presumably qualify as "from the general public".

This seems to me like a "damned if you do, damned if you don't" kind of situation. So what really does happen to the bag, from when passenger and bag are separated, until the two are reunited?

For simplicity's sake, let's assume that what's inside the bag is completely innocous.
This is more a guess:

Think about the rationale behind the rule. A terrorist should not be able to send a bomb on a flight while staying at home. With this rule, a terrorist is forced to sacrifice himself by taking the same flight. Hopefully, most terrorists don't like this idea.

If the luggage wasn't put on board, or was put on another flight, the terrorist will notice this at the destination, and not before. Same effect, he had to be willing to sacrifice himself by boarding the aircraft.

So, the rule is not to never let luggage fly alone, it is about not letting it fly when the owner for some reason does  not take his flight.

If the luggage took another flight, it was for sure an error. The barcode scanner mis-read the barcode, the ID of the trays transporting the luggage was not read correctly, a computer error or whatever. Should not happen, but does from time to time.
If the luggage did not fly at all, it might simply have missed the flight due to technical / logistics problems. Or the barcode tag was ripped off etc.