...Hopefully it's ...a one time thing.
.are you kidding? i'll take a credit every month if its offered! LOL
Not to unduly alarm you, but if you're sure you have no idea what that credit is I would call your CC company ASAP and have them cancel that card and re-issue a new account number.
I have seen instances on the various credit card messageboards of mysterious small credits appearing on folks' statements, only to have their number hijacked and used fraudulently a month or two later.
I think the bad guys will often attempt to place small credits on harvested credit card numbers to ensure the account is valid and active before they start using it.
becareful, i remember something similar but it was about checks being sent out from credit card companies to enroll you in a payment protection plan free for 30 days, and then they charge you, i would call to make sure that you are not enrolled in any such programs.