When addressing a retired fire or police chief, is it proper to use "ret." or "Ret."?
It would be Chief Joseph Brown, FDNY, Ret. as part of an address in written correspondence/documents. Dear Chief Brown, as a greeting in a letter. As a form of verbal address, "Chief Brown", assuming he is among subordinates/peers with whom he is acquainted/has worked in the past.
Hope that is helpful. I'm not a teacher, though.