Races emerging before winning the sector
I was under the impression that there's only one moment where a race could emerge in a galaxy, and that's the exact moment when you win (or lose) a sector. As in when you get the win message, any unemerged races that still exist at that moment will then emerge with the message "The Blah race have now EMERGED in this galaxy!" indicating that they can now possibly show up in future sectors of that galaxy. Once that emerging message shows up, there's no going back; you can eradicate that race before leaving the sector, and they'll still show up in the next one.
Anyway, what I'm getting at is that I've had a few exceptions to the above. The first one was when I got the Cortex-Brunt Hybrid to break out (a second time, the first time they died horribly thanks to being in a system full of space mines). I was already close to winning at that time, so I didn't think much of the CB Hybrid's emerging message appearing right as they showed up instead of waiting for me to win first.
When it happened a second time, that's when I knew something was up. In a sector before I could meet them, the AI race was eradicated by a Zarlok ancient ship and proceeded to claim their planet. And then a Hezog took that planet, and to my horror I got the message that the Hezog have now emerged even though I was nowhere near winning. The Zarlok and Hezog retook the planet a couple more times before I finally found and blasted the planet myself, but it was already too late; I won the sector with neither ancient race alive, yet in the next one I find myself sharing the starting star system with a Hezog planet. The only bright side to this is that at least the Zarlok weren't able to emerge... for now, it seems.
And lastly, the Legion also somehow managed to emerge and I'm not even sure how it happened. Out of the many times they've captured a planet only to get eradicated by me shortly afterwards, did I somehow miss the emerging message? I always made sure that no monster races and ancients have any planets upon winning and even upon leaving just to be sure, so I was really surprised when I found the Legion among the starting races of a new sector.
So, is this a bug or a feature? As in aside from winning the sector, there's also a very small chance that a new race upon appearing in a sector (either via breaking out, being resurrected, or when a monster or ancient capture a planet) will immediately emerge? And also, does this mean that no matter how careful you are, the monsters and ancients will find their way into the galaxy eventually? I mean the Nexium was definitely my fault, but I just can't help but think that the Legion and Hezog cheated their way in.