Are retirees subject to UCMJ?

Yes they can for crimes committed while on active duty.
https://www.washingtonpost.com/world/national-security/in-rare-spectacle-army-court-martials-a-retired-general/2017/08/25/473fc440-89ac-11e7-a50f-e0d4e6ec070a_story.html?utm_term=.bb356b509580

I think this would be the case only with a serious offense on a military base or involving other military personnel.

I have been medically retired from the NAVY for 40+ years and I am still subject to UCMJ?

This artical has kept me anchored...lol

Has happened on several occasions. The MCM is very clear.

Back in The 80's I knew of a Senior Chief Ship's Serviceman brought back onto active duty after retiring because an audit revealed theft from the ship's store that the SHCS had been a part of.

As they should be!

Long after I retired, a gunny came up to me and asked me why I was wearing my Marpat trou. I told him because I could. He first told me that what I was doing was stolen honor. I was going to blow him off, but I decided to show him my ID, and then he said I could get recalled and court martialed for wearing the trou. At that point, I tuned him out despite his continual insistence that I go change out of them.

Long after I retired, a gunny came up to me and asked me why I was wearing my Marpat trou. I told him because I could. He first told me that what I was doing was stolen honor. I was going to blow him off, but I decided to show him my ID, and then he said I could get recalled and court martialed for wearing the trou. At that point, I tuned him out despite his continual insistence that I go change out of them.

I assume so. I have been retired now for the past 23 years and only once I had read about an incident in Puerto Rico. I believe it had something to do with the bombng range that the military was using against the wishes of their government. He decided to return all his personal awards and I think his retired military pay too. But don't quote me on this because the article has been so long ago.