Do you think I need to call the bank back and confirm I am a US citizen or is that opening a pandoras box?
This is really up to you, but I wouldn't worry unduly about it. You will file your information returns to the Treasury or IRS. You've done all that is required on your part. The bank has an obligation to fulfil their part, and how they do that is their business. Be sure to record for any future reference when and what the call was about, what they asked, and how you replied.
I would be careful of any unsolicited calls one receive regards FATCA, account holder details, etc., as there are scams around as a result of FATCA. There are warnings about this on the IRS site.
We're probably going to see a great deal of inconsistences between requests from the various banks. Those that use the same 'FATCA Advisors' will respond similarly, and those that use others, or none at all, will carry out their search for US Persons differently.
Respond to your bank/building societies demands. There is no need to offer additional information because you know more about FATCA than they do. That's their problem. Just be sure you've covered your obligations and responded truthfully to the request for information (provided you're certain it is
your bank making the request).