Mysterious Banking Deposit

[HejiraNYC]

Has this ever happened to you?

I happened to check my banking accounts online this morning (as I do about twice a week), and I noticed a new deposit sitting in my checking account. A pretty big one- over $4000. I have NO IDEA what this is, or where it came from. The source is rather cryptic to me: "(name of my bank) NFSC CPA CREDIT BALANCE PPD ID: #########." I don't think this is coming from my brokerage account (which re-invests any gains/dividends automatically). I am not expecting any tax refunds. It's certainly not from my work. What would you do? Be quiet and leave well enough alone? Alert the bank to a potential mistake, thereby losing this mini-windfall? Any theories on what this could be?