I have often wondered about this. GLA domestic reclaim used to be completely open to anyone to walk in, as did the old LGW South domestic bit downstairs. For my part, as well as not packing anything valuable, I always at least position myself where I can see the bags coming on to the carousel.

But... there have to be much better targets for petty thieves than baggage reclaim, which presents a fairly poor value proposition:

a) Luggage, contents unknown, but likely to be dirty underwear;
b) An area where the owner is likely to be there looking for his bag, and therefore a good chance of being caught red-handed;
c) Somewhere covered in CCTV, and guaranteed to have a heavy police presence.

TBH, my biggest concern is a genuine error by someone else with a similar bag.