It's rare, and I work with a bunch of retired Army guys (I are one). I pheasant hunt with a guy that refers to himself as "The Colonel" and most guys call him that as a term of affection. We've also got a few of "Rangers" and Sargeant Major's (Ret) running around post.
Most of the guys using rank and (ret) afterwards are GOs unless it's in a form of official correspondence. The organization I support employees a bunch of "Senior Mentors" that are normally retired GOs and always they're caveated with the (Ret) either post name, or post rank. Some guys are funny, I've worked with 3Stars that retired and turned into "Bob" while others are still clinging to "Sir" or "General".
Honestly I don't see an issue with it, the guys earned it.