This is based on games started before the update on Friday, February 12.
In one game I have been playing I am the Terrans, and the Snathi (the major race) declared war on me. I didn't know where they were, so I hired the Mercenary scout ship "The H12-Super" to look for them.
I found them but was careless and stopped too close a Snathi ship, and the Snathi ship attacked the Mercenary ship. I chose Quick Battle and watched to see my Mercenary ship blow up. It didn't. Instead, it changed color to the Snathi color, then I lost vision in that area. Later, I saved the game, quit GC3 and restarted with cheats enabled so I could clear the FOW and see if the ship was still around. I found it scouting for the Snathi.
So the Snathi can capture Mercenary ships. This could be bad if you are at war with the Snathi or good if you are the Snathi.
In another game I have some minors in my territory. One of them, the Odair Conclave, has been sending ships to destroy pirate ships being built at a pirate shipyard in my territory. During one turn I noticed that one of minor's ships had an odd icon. When I looked more closely, I realized it was a Mercenary ship.
I wondered if they hired it or if it was given to them by some other race. I checked the Bazaar and found that they hired it, then I found that they had hired another Mercenary.
Now you know that you have more races that can hire the Mercenaries than you thought.