That's a very particular case: if you know or should have known that money deposited into your bank account isn't yours and you take it out anyway, it's stealing from the bank and you can go to jail for it. This has happened to more people than you might think (although usually for more than $200). Nobody's going to jail over extra FF miles that just appear in your account.

And, I do think an earlier poster's claim that ethics are like being pregnant is overly simplistic. While one should always have ethics, it's not always clear what the ethical thing to do is.

If it were to happen to me, I would write a letter to the frequent guest program giving the facts as you know them and then be done with it. If you don't hear back from them, and a reasonable period of time passes (perhaps 6 months or whatever amount of time the program allows for missing stay credits), I'd consider them bonus points and spendable.