In a mantis controlled sector, there was a mantis ship that jumped away ridiculously quickly. Normally there's plenty of time to take down the engines, but I didn't even get a single shot off with a 10 second weapon (with level 2 crew and two automated preloaders, probably about 5 seconds) before it jumped away with my two favorite boarding Rocks.
It gave an option to pursue it, so I chose that and got a quest marker, so this was obviously a special event..
I beat the ship that I found in the quest, but my missing crew was nowhere to be seen
I think if you're able to track down and beat this ship, you should be able to get back the crew that they took.