REAR END

[Chamberpains]

Without a picture of it, it is almost impossible to definitively say what it could be. There is a lot of moving iron in a rear end. If it was just a single piece of metal like that I would take note of it and not get terribly concerned. But I would go by a cork or another plug that fits your drain plug and your magnetic check plug and begin pulling them out and checking them frequently for a while looking for more pieces of metal. The Cork and the extra plugs are to swap them out real quick so you don't have to drain all the gear lube out while checking them.