Retired/separated federal law enforcement officers fall under the Law Enforcement Officer’s Safety Act of 2004 as amended (LEOSA). This is often referred to as HR 218 but the true citation is 18 U.S.C. 926C.
926C allows a “Qualified Retired Law Enforcement Officer” (QRLEO) to carry nationally, subject to certain restrictions as outlined in the statute. There is another thread where there is additional information on LEOSA and also has a link to a USCCA article on the issue.
One thing to be very aware of is that LEOSA does not allow QRLEOs to carry anywhere unless there is a specific federal or state statute that would allow it and it does not extend anyone’s statutory power of arrest or any type of immunity generally available to LEOs.