I have had these things happen before - do you call and check on it? Do you say nothing, fingers crossed that you get extra hard cold cash?
Am I a sucker and should have kept my mouth shut?
Call and check on it. Most of the time, corporations are not going to give you the benefit of their mistake.
When I was unemployed back in 2009/2010, I found a weird deposit of @$1000. No real answer on why the money was deposited to my checking account (likely the bank teller typoed the checking account number during the deposit transaction at the bank).
They immediately retracted the money when informed. They didn't just go, "Our mistake. You can keep the money. We'll just reimburse the correct checking account. It'll be a win/win for all involved."
I think technically if I used the money, it could be construed as theft.
They don't care about anyone. They just want their money and worse... even if they were 1000% in error, they'll still hold you to late fees and more unless you're superlucky to find a near-extinct level rare manager with a sense of compassion and empathy. Even then that means you won't have to pay a late fee or whatever.