"That's the old duke's banded mail!" rule
Whenever a magical weapon or armour is discovered, there is a 10% chance per bonus that it is a "known" item and the original owners, or (more likely) their descendants, are still in existence. The DM should pick a suitable candidate in the region for ownership. Whenever the PCs visit any settlement within 50 miles of the location of this original owner(s), there is a 1 in 3 chance per day that somebody in that settlement will recognise the magic item and report it to the owner, unless it is kept covered.
On discovery, the DM should roll a d6 to determine the owner's reaction:
1 - Jealousy. The owner will attempt to regain the item however he can.
2 - Anger. The owner will attempt to regain the item and, preferably, punish the PC.
3-4 - Vengeance. The owner will attempt to kill the PC and, preferably, regain the item.
5 - Appeal to authority. The owner will attempt to bring the PC to justice in front of the local ruler as a "thief".
6 - Plea. The owner will attempt to contact the PC to buy the item.