offshore bank customers

Some really good questions. As someone who lived and worked overseas from 2002 - 2011, then retired in 2017 to Thailand, I have experience with reporting my overseas personal and business financial accounts to the IRS in the US every year I met the requirements to do so. If you have the equivalent of US$10,000 or more in the aggregate for even only one day during the calendar year, you are required to report all your overseas accounts and the highest balance during the given year on the IRS FBAR form.

As far as I know, when foreign banks became required to report their US citizen and US green card holder bank accounts to the US IRS starting in 2013 per the US Foreign Account Tax Compliance Act (FATCA) passed by the US Congress in 2010, the information flow was between the foreign banks and US IRS.

Whether or not this information also gets shared with other organizations such as the OECD, etc., I don't know. You can Google the name of a foreign bank, asking if the bank is FATCA compliant and you may be able to know if your overseas bank is compliant with FATCA.

Regardless of whether or not you report your overseas bank accounts and income on your US tax return and are in compliance with FATCA, the foreign banks don't know that and just report their US citizen/green card holders accounts to the US IRS as required.

Hope this helps.

Matt M.