Since something like this has occured to me on more than one occassion, and in my life two rules always prevail.
1. No good deed goes unpunished.
2. Whatever can go wrong, will go wrong, and fuck me at the worst time.
I of course opted to track down why in the hell I was getting what I thought was free money credited to one of my accounts. I would request copies of all payments including copies of checks etc. until I found out how and why this happened.
More often than not it always came back to the same damn reason, it was never a mistake, nobody paid for me, and yes the money was mine. The most common reason's being that at some point either overcharged me at some point and then found the error and credited me, or #1 reason when I opened the account they took a deposit that was to be held for so long and after I paid my bill ontime every month for 2 or so years they refunded it to me in the form of a credit.