It’s been well known to lawyers practicing in military justice and at least to some reservists and retirees (thanks to same cases with news coverage) that you can be recalled to active duty for crimes allegedly committed while on active duty. (Actually, technically a retiree doesn’t have to be recalled, he or she can be court-martialed in civilian clothes!) The same goes while you are on the IRR (Individual Ready Reserve), which is why someone who hits their ETS before eight years of service is still subject to recall for court martial until they hit the eighth year. (But if you get a punitive discharge that prevents you from going to the IRR then you are not subject to recall; sometimes something to keep in mind when facing a separation.) In fact, I had one client who was in the IRR and was at 7 years and 11 months when he was recalled to active duty to be tried by court martial for forcible rape! (Thankfully I defended him aggressively and we got a full acquittal.) One more month and he would never have been tried.
The same ability to recall for court martial applies to military retirees. Perhaps the most famous case was that of MSG Timothy Hennis. While serving he was acquitted in a civilian trial for rape and murder. He continued to serve up until retirement. After he retired, the military recalled him to duty and court martialed him for rape and murder using DNA evidence that could now be tested. (Generally, double jeopardy doesn’t apply when you were acquitted by a state government, you can still be tried by the federal government or military.) He was convicted and sentenced to death. (If he had left the Army before retirement then he never could have been tried again.)
What most retirees, and even many lawyers, don’t know is that the UCMJ still applies to retirees even for acts while in retirement and everywhere in the world. This is due to Article 2(a)(4) of the UCMJ. It also applies to medical retirees. This is as crazy as it sounds. If you are retired, you are subject to court martial for using “contemptuous words” towards the President or some other government officials, smoking weed, adultery, conduct unbecoming an officer (if you are a retired commissioned officer), having sex visible to any third party, and many many things that civilians can legally do. The legal justification for this is that your retirement checks are actually a form of pay for potential future service. Never mind that the IRS and family courts (if you get divorced) treat that money as a pension for past employment. Yes, one part of the government treats it as current wages (so they can screw you by court martial) and the other part treats it as a pension (so you can be screwed financially).
Realistically, it’s rare for a retiree to be court martialed for acts committed as a retiree (they probably don’t want the media to know this is possible). But in a recent case a retiree was tried for possessing child porn as a retiree (not while he was on active duty) and convicted. The military courts upheld his conviction. http://www.caaflog.com/2018/01/01/top-ten-military-justice-stories-of-2017-1-exercising-court-martial-jurisdiction-over-retired-members