We mail the AAN to the address provided on the application. If it is returned we verified it was mailed to the address provided. If it was then we put it, along with the envelope stamped by the PO as being undeliverable, with the application and maintain it for the 25 month retention period. We make no other due diligence attempts in locating the applicant.
The opinions expressed are mine and they are not to be taken as legal advice.