It's not that I'm going to pretend that I know for sure it's not true - but it is like the old argument "God works in mysterious ways". If there's no metric we can refer to because the Big Bad Corruption happens where we can't measure it by definition, then how do we know? How can we test our suppositions? Would we be able to recognize an honest administration if we saw one?
Put it this way: if there were an undetectable amount of stealth corruption in all administrations no matter what, then what does it tell you when we have a certain administration that gets pulled up on their dirty laundry more than the other ones regardless?