No I think it's you who misread what I said. I never made a distinction between internal project developers or external folks making these checks, and my assertion still stands - even for open source projects, with the entire OS world having visibility, the vast majority of software goes unchecked. For a few major, high-visibility projects then I'm sure there are, but for the majority - no way.
Ah ok, Maybe I misread.
For the vast majority of open source code, I will fully agree, that it all does go unchecked. However, the OP was asking about DISTROS, and not individual projects.
So,, I do disagree with you in that the code is never checked. Again, I'm not talking about all open source, but in the context of Distros, and not individual projects, which is pretty much what the OP was asking, they are released, and other organisations / companies get hold of these distros and they themselves will look into that distro to look for any thing, any code, to help them to improve their own distro.
This way, these distros are in a way, self regulated.
Many companies have coders, who are there purely to examine other distros, certainly the bigger companies do. Apple and Microsoft hire hundreds of them, just to rip apart other code to find some good code that they can use, or even just for ideas, and there are a fair few Linux companies now who are doing the same. SuSE and RedHat are two that also have dozens of programmers and hackers who are employed purely for that purpose .
Not just that, but globally, there are thousands of people all over the world ripping apart code, be it open or not, to get at anything that they can use, even if its for rotten purposes, but if they found out something nasty, it will soon get leaked.
Again, this also means that if a distro does have something unwanted, or downright nasty, they wont be able to hide it for long.