Oh! Ummm.....if you were worried about that I think a trip to the vet is the best idea, magnets can cause serious serious damage inside an animals system as if strong enough it will try to move through tissue.
The only thing I would say is it is pretty hard to remove the magnets they are normally wedged and/or glued in there pretty solidly. If the leg is chewed then it possible otherwise I am not sure how else your pup could have freed the magnet.