A bag that fails to arrive is rarely lost in any permanent sense. It enters a standardised tracing system that reunites the large majority of separated luggage within a few days.
Separation usually happens at a known point
Most delayed bags miss a connection rather than disappearing, because the bag's transfer window is shorter than the passenger's and the sorting system is sequential.
Others are offloaded deliberately when an aircraft is weight restricted, or fail a scan and are diverted for manual handling.
In each case the bag remains within the airline system with a record attached, which is why tracing usually identifies its location quickly.
The tracing system works on descriptions
When a bag is reported, the details are entered into a shared industry database used by airlines and handling agents worldwide.
Unlabelled bags found anywhere are entered into the same system from the other direction, described by type, colour, make and contents.
The system matches the two records, which is why the description given at the desk matters more than most travellers assume.
Delivery is arranged, not collected
Once located, the bag is normally forwarded on the next available service and delivered to an address by a contracted courier.
The passenger provides an address for the coming days, and that address can usually be updated if the itinerary moves on.
Delivery to a moving itinerary is harder, which is why the system asks for a stable address wherever one exists.
Interim expenses follow defined rules
Airlines are generally obliged to cover reasonable essential purchases while a bag is delayed, though what counts as reasonable is defined by the carrier and by applicable law.
Liability limits for delay, damage and loss are set by international conventions and by national implementation, and both the limits and the claim deadlines vary.
Because those rules differ by route and change over time, the applicable terms and time limits are worth confirming for the specific journey rather than assumed.
What reduces the risk
Removing old routing labels prevents a scanner reading the wrong destination, which is a genuine cause of misdirection.
Contact details inside the bag allow identification when an external label is torn off, and an internal label survives handling better than an external one.
Carrying medication, documents and one change of clothing in the cabin removes the consequences of a delay that the tracing system cannot prevent.