How to retrieve checked-through luggage if I don't board a connecting flight?

I did this flying from London Heathrow to Calgary. I was supposed to fly on to Vancouver as part of my itinerary. Then, back to Calgary (home). At Heathrow I simply asked the checkin to short-tag my luggage to Calgary as I would not be taking the Vancouver flight. There was no issue at all and my luggage was taken off in Calgary. I was told you can not be made to board a connecting flight or any other flight you have purchased. It was different outbound. I had to Fly Calgary to Vancouver then back to Calgary and catch the same flight I would have in Calgary to Heathrow. So silly. You have to take the initial portion of your itinerary or it would be cancelled but not so for me on return. Hope this helps

Answering your second question first, if you are travelling A-B-C and want to retrieve you bag at B, you would ask to short-check your bag to B. Airlines have varying policies around this and whether they will allow you to do so. This may depend on the connection time at B, whether you have an overnight stay, or whether you’re on a fully flexible ticket. Check-in staff may be able to make an exception to their policy if you give them a convincing enough reason, such as you have medicines or medical equipment that needs retrieving at B, or you are delivering something to someone at B. The reasons they have policies against short-checking have been explained elsewhere and are for things such as discouraging hidden-city ticketing.

If you arrive at B with bags checked to C and you don’t board your connecting flight, the rules depend upon the country you’re currently in at B and whether B-C is an international flight. As Doc mentions in comments, Positive Passenger Bag Matching rules apply to certain flights.

Positive passenger bag-match (PPBM) is a security measure aimed at
preventing baggage unaccompanied by passengers from traveling in
aircraft luggage compartments. Such matching is now performed on
virtually all international flights

On domestic flights in the USA this is only one option that currently may be selected if explosive detection systems are not in place.

(e)Mandatory Screening Where EDS Not Yet Available.—As soon as
practicable but not later than the 60th day following the date of
enactment of the Aviation and Transportation Security Act and until
the requirements of subsection (b)(1)(A) are met, the Under Secretary
shall require alternative means for screening any piece of checked
baggage that is not screened by an explosives detection system. Such
alternative means may include 1 or more of the following:

(1) A
bag-match program that ensures that no checked baggage is placed
aboard an aircraft unless the passenger who checked the baggage is
aboard the aircraft.
(2) Manual search.
(3) Search by canine
explosives detection units in combination with other means.
(4)
Other means or technology approved by the Under Secretary.

If PPBM rules apply, the airline has no option but to unload your bag if you choose not to fly or do not board. If you inform them as soon as possible, this will allow the airline to remove your luggage without causing a departure delay, which failure to turn up at boarding time almost inevitably would. Theoretically you could inform the airline at A that you have no intention to board at B and that they would inevitably have to unload your luggage anyway, but that would create the risk that they would refuse to let you fly without repricing the new itinerary, or charging for short-check, e.g. on Virgin:

Our bags could be checked to LHR only for a fee of $215 per pax! Half
an hour later and $430 lighter, our bags are checked only to LHR.

Others on that thread have managed to get their bags short-checked without fee.

An alternative to requesting short-check is to find an itinerary that requires you to retrieve luggage during transfer. One common method is to require a change of airport, such as LHR-LGW, LHR-LCY, JFK-LGA, CDG-ORY which is an option if the transfer city has multiple airports. Another option is to force an overnight stay. e.g. from KLM’s policy:

When transferring, you only need to collect your baggage and check it
in again for your connecting flight, if:
…
– You arrive at one airport and your connecting flight departs from another airport
– Your transfer lasts longer than 12 hours or your next flight departs the next day.

If the airline refuses to allow short-check and you arrive at B, you would inform the airline staff that you are not flying B-C and request that they retrieve your luggage. This is not at all an unlikely event as people fall sick during journeys, get lost, distracted or delayed during connections, and may need to change travel plans for other unexpected reasons. Obviously, airlines must have procedures for removing luggage in certain circumstances. Assuming the airline agrees to retrieve your luggage, there may be a considerable delay in returning it to you, and it may get lost or redirected during the process. The bag tag will still have final destination on it and there is a risk that other processes in the system result in the bag not making it to the luggage belt. Some airlines reportedly charge to retrieve your bag in this situation:

After a lot of run around to recover my bags I ended up at the ticket
desk, where I was charged €275 each person for the luggage recover! By
now it was too late to go with the bags to London, so other than
abandoning the luggage I had no choice but pay 550 euros , to get my
luggage.

And Air France (closely related to KLM):

Air France says the only other way to get my bags at CDG is to pay a
€275 fee once there, as there’s a policy for check-in agents NOT to
honor requests to tag bags with the connecting airport.

Air France general conditions of carriage:

If the Passenger does not use all their Flight Coupons and prematurely
interrupts their journey, the Passenger may be required to pay a fixed
amount, specified by the Carrier when the Reservation was made, in
order to be able to retrieve their Checked Baggage.

Given a lack of itinerary and airlines in your question, that is all that really could be said. Each airline has different policies and procedures that may also depend on the itinerary and transfers, and you would need to request short-check at check-in and negotiate/escalate. In general, it would be best to check the airlines’s conditions of carriage for any specified fees, e.g. for British Airways

3c2) Your ticket is no longer valid if you do not use all the coupons
in the sequence provided in the ticket. Where you change your travel
without our agreement and the price for the resulting transportation
you intend to undertake is greater than the price originally paid, you
will be requested to pay the difference in price. Failure to pay the
price applicable to your revised transportation will result in refusal
of carriage.

Providing you are skipping the final segment, failure to pay any additional fee for the journey as flown only results in the refusal of carriage. There is no mention of any charge to retrieve luggage.

http://www.flyertalk.com/forum/25720254-post7.html

There’s no requirement to give a reason for reclaiming bags other than
“I am not flying to CPH”. However a change of plan and an onwards BA
ticket would be fine. It could involve 2 hours of waiting however.

Similarly, I see no mention of a fee in American Airlines or Alitalia CoCs.

As for real-life experiences of people who have not short=checked and retrieved baggage before final segment, there does not appear to be much in the way of anecdotes of this situation. Will update answer following further research.

How and when can I retrieve my luggage at B? (Or if not, what is the fate of my luggage?)

Definitely when overnighting in B. While sometimes the airport might be willing to hold your baggage overnight the opposite is the regular practice: if you overnight you not just can get back your baggage but most often you must collect your baggage. (If you want to do a hidden city ticketing using this then ITA Matrix has a minconnect option.)

As soon as you know that you have to do this, go immediately to a representative of the airline and explain your situation. Be prepared give explain in detail why you need to terminate your journey early. If this is a genuine emergency I would expect that they would make every effort to retrieve your baggage. If this is just you changing your mind on a whim, or it looks as if you intended to finish your journey early I would expect them to be much less cooperative.

There is a significant chance that your bags will be allowed to fly on to the final destination and then returned to you later – at least with a US domestic flight and if the transit time is short, or you simply fail to show at the gate for the second leg. US domestic flights do not always unload baggage if a passenger does not show. In any case it is unlikely that they will simply unload your baggage to the carousel for your arriving flight. You will need to contact the airline to retrieve your baggage. If the airline is forced to delay the second leg in order to unload your baggage, expect to be extremely unpopular both with the airline and the other passengers (although the passengers won’t know its you, so you will be unpopular only in the abstract).

I’ve never heard of it happening, but in my opinion I can’t see why, if your bags make it to the final destination, the airline shouldn’t charge you to have them returned.