I've known people who "lost" their horses on a trail ride.
Still can't quite figure out how it happened, except that I do know those riders tend to be a bit careless (is "stupid" too harsh?) when it comes to their riding and tying. The horse took off without them and was never seen again. . .nor was any of the brand new Tucker tack it was wearing.
Quite likely, some local found a VERY nice horse with some VERY nice tack and decided to keep it. While it would be nice to think that the horse community is made up of people who are honest and willing to help each other out. . .there are those who figure "finders keepers, losers weepers" and keep whatever they find.