Can some still be considered a [RANK] after exiting the military?

Yeah, I know quite a few retired/separated military people of various ranks- the officers generally got out between O-3 (Captain/Lieutenant) and O-6(Colonel/Captain), and the enlisted guys got out generally between about E-4 (Corporal/Specialist/PO3) and E-7 (SFC/Gunnery Sgt/CPO), and with a very, very few usually joking exceptions, NOBODY calls them by their military ranks.

I mean, the guy two levels up in the corporate hierarchy is a retired USMC Colonel, and nobody calls him “Colonel”- it’s just his first name. Nobody calls my Army E-5 and O-3 buddies “Sergeant” or “Captain”, least of all either of them (they became friends in the Army actually). And none of the WWII or Vietnam vets who were fathers and grandfathers when I was growing up were ever called by any kind of military title, despite nearly everyone’s father having served in one branch or other for some period of time.

The only case I can think of was when we used to play board and video games with a guy who was still in the Naval Reserve and was an E-9, so every now and then when we were playing first person shooters, someone would bust out the “Master Chief” title to awe the teenagers.

Or a chief warrant officer in the US Army.

Chief Petty Officer in my case.

That’s because (like the link above says, though it’s a hard page to read) technically British army ranks allow people of Captain and above to use the title, not Major as was previously stated. It was sometimes considered low-class to do so but it wasn’t illegal. So Captains Peacock and Hastings were fine too.

It may have been significant that Captain Hastings was invalided out?

Possibly. In the first book he was recovering so he was still a Captain.

Also he was working as a sort of assistant to a private detective where his rank might have been slightly relevant. Christie knew about ranks (her brother was a bit of a black sheep except for his military service where he did well) and Hastings was a buffoon most of the time, so it’s possible she intended him using Captain as a bad thing, but nobody in the books reacts that way. And I’m not sure many people used his rank in the later books - he’s in the books less frequently than the TV adaptations.

In any case, there would have been no law against him referring to himself as Captain.

Chief Master Sergeants in the AF are also addressed as ‘Chief’.

Personally, even though I was in the Air Guard for 9 years, I don’t even claim to be a veteran on the occasional form where that’s asked. They usually ask that because having veterans’ benefits may change something about whatever I’m applying for and I don’t qualify for any benefits.

As Elendil’s Heir’s link notes, in the Birtish army

The Brit’s being desperately class conscious love their titles, no matter how humble. Captain Peacock was of course a mere Captain to make the point that he was a strutting no-one. David Croft, who wrote scripts for Are You Being Served also scripted Dad’s Army and It Ain’t Half Hot Mum - both of which satirised the British class system and military, had a very keen eye for this.

An anecdote.
My grandfather was a Captain in the British Army in WW1. On being demobbed he took holy orders, and thus was henceforth addressed as Reverend. He become the vicar of a tiny hamlet in Devon. Sometime later a retired man came to live in the village. Being a village, gossip was rife. So when a few letters came in OHMS envelopes addressed to Major XXX it soon became known around the village, and before long he was being addressed as “Major”. Mornin’ Major, evenin’ Major etc. When WW2 came my grandfather became captain of the local Home Guard (shades of Dad’s Army.) He of course got access to the military records of the village folk. The “Major” had no such rank. He had simply sent himself the letters when he visited London, with the explicit intent of sparking the idea he held the rank. The Brits are funny folk when it comes the class.

Ralph Houk (one-time manager of the Yankees and Red Sox) was commonly addressed as “Major” by sportsreporters, to the irritation of their younger colleagues). He was a highly decorated officer in WWII, though what that had to do with his baseball career was another matter.

My girlfriend’s uncle is a retired Air Force Major General. I call him Bob.

Love Ralph Houk. When asked if he was going to be nervous managing in the World Series he replied “Why is someone going to be shooting at me?”

“I keep telling you, my name is Harold.”

I’m an Army retiree. I can call him whatever I damn well please.

That’s a view of class from many decades ago, though. Class is still an issue here but it is in every country too, including the US. The vast majority of ex-soldiers don’t use their titles these days. Rhe musician James Blunt, for example, is Captain James Blunt but he’s only ever referred to as that when talking about when he was actually in the military.

Seconded. I’m an Air Force retiree. My first cousin is still in, and is a Lt Col. I’ll call “Joe,” just “Joe.”

Joe is a complete tool though. Joe likes to show up for free dinners at Grandma’s house. I try to bring a plate for the crowd. At a minimum, I bring good beer for the rest of the Gentlemen of the family.

Tripler
Joe just shows up for the freebies.

When I was in college, one of us dorm rats had been in the Navy for eight years, got out, and was attending on the GI Bill. He had on his door

[name] BM1 USN (ret)

but he meant it as a joke.

ETA: I missed the edit window. Joe will continued to be called “Joe” after he exits, despite the fact that he will retire at a higher rank. It has been, and always will be, just “Joe.” And I think Joe will resent this for a long time.

Tripler
Joe can suck it. And I will tell him so.

I bank with USAA, and when I have to speak with their call center, they still address me by my highest (officer) rank attained. No idea if that’s actually proper.

Oh and my high school history teacher was a command sergeant major.

Once I was experienced enough to understand what that meant, I always called him ‘Sergeant Major’ after that. More of a respect thing, though.

It’s a courtesy, but nothing more. They do the same thing with me.

Trip

I’m not clear that he did that. Poirot always introduced him as Captain Hastings, but Poirot was a foreigner.

(The few separated-not-retired military types I’ve known didn’t use their rank. The reason they separated was because they didn’t want to be in the military any more.)