I'm adding this excerpt from Interagency Guidance promulgated by all of the functional federal agencies regarding Regulation P and its sister regulations merely to round out the thread:
I.3. Q. I offer consumer checking accounts. I notify my customers that, among other things, I make disclosures as permitted by law. My checking account customers deposit checks made payable to my customer but drawn on a financial institution unaffiliated with me. My practice is to write my customer's account number on the back of the deposited check to facilitate its processing. The check itself then goes to the maker's financial institution, with my customer's account number on the check. Is this a disclosure of nonpublic personal information that would be subject to opt out requirements or the prohibition against sharing account numbers?
A. No. The opt out provisions do not apply to disclosures in connection with servicing or processing a financial product or service that a consumer requests or authorizes. Nor do they apply to disclosures that are required, or are a usual, appropriate, or acceptable method in connection with settling, processing, clearing, transferring, reconciling or collecting amounts charged, debited or otherwise paid. §§ 332.14(a), 332.14(b)(2)(vi)(A). Also, because the account number is added to the check solely for use in processing the check and is not used in connection with marketing by a third party, this disclosure is not prohibited by the ban on disclosing account numbers for marketing purposes. § 332.12.
I'm not suggesting that the GLBA privacy regulations dispose of the issue because they do not. All they add is insight. The wisdom inherent in the bank adding the depositor's account number remains a judgment call, but it is clear that the bank adding the number should have more than a casual need for its presence and that a trace number is far preferable.
There are other threads that discuss writing the payee's social security number to the back of the check. That is not what is being discussed here. I think that practice is plainly irresponsible.
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In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.