Unexpected $42,400 bank deposit

Out of the blue, my brother found a totally unexpected $42,400 deposit to his bank account. Then a day later a $2.60 service charge for the transfer. He is alert for the possible fraud of the sort where someone calls or emails with a desperate request to send them cash or they’ll lose their job. No one like that has contacted him yet and I don’t think he’ll fall for it, even if the person is “from the bank”.
He also called the service department at the Giant Monster Mega Bank and shuffled among customer service to get a knowledgeable person. That guy said it could have been an account number typo from a sender at the sending bank. (Apparently they don’t need a name match also?)
The customer service person said the only solution was to show up in person at his bank branch with his ID and that person could then reverse the transfer / correct his account. One complication for him is that it’s 50 miles to the nearest branch since the GMMB just closed the branch office which had been close to his home. So it may be a month or 2 until he happens to travel to a branch for another reason. Anyway, if it’s not a scam, I’m pretty sure the sender will be able to claw back the transaction by then.

Has anyone else heard of something like this?