For example my brother lost a vessel in a hurricane - he was bunkered down on the mainland and it was gone after the storm - we assume the storm surge lifted the anchor off the seafloor and it floated to who knows where and wrecked.
It wasn't insured, so wasn't reported. I suppose it's possible someone stole it, but I can't imagine anyone would be brave/stupid enough to do that during a hurricane.
Maybe someone somewhere in the world got lucky enough to see a drifting abandoned vessel and claimed it... but that's pretty unlikely.
Forgive me if this is a dumb question, as I'm not a sailor, but wouldn't one want to report it as at least "missing" since it does have registration? I could see that having at least 2 benefits: if spotted, they can return it, and if involved in something nefarious your brother would have a solid alibi of why it would be in someone else's hands