From CA:
We implemented a new letter, FTB 3904, Request to Confirm Tax Return Filing. This letter will put the potential victim on early notice. The letter asks the valid taxpayer to notify us whether or not they have filed a tax return. We only send this notice if the tax return looks highly suspicious. We send the letter to the last address we had on file for the taxpayer, not to the address on the suspicious tax return.
https://www.ftb.ca.gov/Archive/Professionals/Taxnews/2014/December/04.shtml
