A family friend un-retired the last time they did something like this (~2009?). He retired a few years prior, but I don't recall if he was working another job on the outside, or if he was simply enjoying the retired life. He was a flyer (mission crew, not a pilot), and he did go back to flying. He went into a flying training unit, teaching the new guys how to do the job. He did that for a couple years, then was back out again. He was happy with the opportunity, and enjoyed the extra time as an instructor, which is mostly what he was doing before he retired the first time.
His situation is the sort that this kind of program is great for. He loved what he did, and bringing him back in as an instructor alleviated the need for an "actually active" person working the the training squadron, who could instead be flying the real missions overseas. Plus, as an old head, he had lots of experience & perspective that was valuable to pass along to new folks.