[LEAKYSEALS951]

I wonder if the cat lady who tagged him has his brother.
I could see a situation where both the cats befriend a neighbor, who "adopts" them. Perhaps a cat lady, perhaps a family with children who adore the cats. One of the cats stays, and the other returns home. Since the one that returned to you was kept indoors for safety reasons, the new "owner" considers the cat "missing" and swore if it ever returns, she would adopt the brother as well to keep them together. This would at least partially explain the behavior and tagging. The family was "waiting" for the pet, and when it "came home" they either already had the tags, or were ready to immediately pull the trigger and go buy tags for the beloved missing sibling. If a dispute arises, check local pet ownership laws in your state.
Could be totally off- just a thought. This is how my wife adopted the dog snoring next to me right now!