If you have any documentation showing your retiree status, I recommend that you bring it with you whenever you wish to rent using your former employer's retiree rate code. The major rental-car companies are now taking steps to cut down on fraudulent use of favorable rate codes, and are within their rights to require proof of your entitlement to use those codes.

I recently rented from Hertz at my local airport, utilizing my alma mater's "leisure" rate code, which both employees and alumni are entitled to use. At the Hertz counter, I was asked for proof of my affiliation with the university. Fortunately, I had brought along a certified copy of my university transcript, showing the conferral of a degree. (I had also been asked for the same when renting from Avis/Budget at the same airport, again utilizing my alma mater's leisure rate code for Avis/Budget.)