Found it again. It definitely came from the rear quarter of the animal, though it was impossible to tell from exactly where. But now that I've got the area narrowed down I can pay more attention next time.
You have there what is the rabbit version of the "knee cap". Even when you bone them out it is so small and surrounded by flesh they are easy to miss. If it bothers you just remove the meat on the front of the middle leg joint. Half inch either side of the joint should easily take out the bone. I am used to wild bunnies and my experience is limited. But usually the rear leg is so small from the "knee" down the section is not worth saving and gets discarded. So no trouble with the knee caps.
That makes perfect sense. I cut them off the hanger right at that joint, which would also explain why its only sometimes I find this bone. I'll move up the leg a little, and let the misses know too.
Funny you mention there's nothing desirable below that joint... I have a dog who eats anything, including months-old deer bones she found in the pasture this evening, as well as a few pigs who are not slouches in the dispose-all department. None of them will eat the foot!