Mary, Rick,

First of all, the check could have been deposited to your account in any of your bank's branches. In fact, I believe that a big branch is more likely to be used then a smaller one.

There is a standard settlement procedure used by financil institutions to handle mistaken wire transfers and similar situations.

The short version: if you receive money that is not yours, the ONLY thing you do is authorize your bank to return the funds to the sender under the reference "unable to apply the funds". Nothing more.

Details:
Since the money was credited to your account by your bank, they are THE ONLY ONES who can ask for authorization to reverse the credit and return the funds to the sender. Your bank will do this (and nothing more) only after receiveing a similar request from the sender's bank, which, in turn, receives a similar request from the sender.

Any related message exchange MUST FOLLOW EXACTLY THE SAME PATH of the payment itself and MUST travel through autentified message exchange systems, preferably via the payment system itself.

Examples:
1) Your bank shall not accept any instruction from the sender, only from the sending bank.

2) No faxes, emails, phone calls or the like. All requests for devolution of funds, inquiries and informations are sent through SWIFT or other safe comunication system.

A new scam:
The local branch of Lloyds cautioned customers against another scam using a real deposit and a real check. It could happen anywhere:

Someone shows up and informs you that he deposited a small amount to your account by mistake. He produces the deposit ticket and aks if you would kindly write him a check to return the funds. You verify your statement. The money is there and was deposited in cash, so you give him the check. Then he erases the amount, replaces by a larger figure and collects.

What went wrong?
He accepted instructions to act from someone other then his bank and returned the funds directly, bypassing the bank, when the proper procedure would have been to follow the original comunication and payment path, as described here.

Whoever makes a mistaken deposit to your account must contact the bank (not you) and the bank will contact you. If you feel compelled to do something fast (honest people usually do), just inform your bank that you found an unknown entry in your statement of account and request and authorize the bank to "take the necessary steps to reverse the entry". Be carefull not to mention names or phone calls. The bank will verify the documentation of whoever claims to own the funds and the bank will return the money safely. You only talk to your bank.

Take care,
Luiz