I’ve never seen that before. Did you see the actual solid spur left behind in the cage? Or was it the covering of the spur. That comes off with the rest of the outer layer of skin when they shed. You occasionally see two little tubes attached to the shed skin on either side of the vent.
Anyway, if one of the spurs really is missing now, and if you intend to use him to breed, he’ll just have to use the other spur in courtship. A couple of my older males have worn their apparently preferred spurs down to nothing. They still do fine with the females.
-Joan