Likely on this basis:
https://www.bbc.com/news/technology-35038971.amp
And since the transaction history is traceable, you cannot posit mysterious other transactions that are unaccounted for. Those would be quickly revealed.
> Wouldn't that, in fact, be evidence of a system that is successfully punishing corruption?
I think that's the OP's point? The system (successfully) invested a lot of effort tot punish the corruption.