Yandex is a public company with stocks on NASDAQ. Can you ever imagine how it is hard to hide bribes inside of public company budget? :)
Let's say someone claims that their programmer genius Ivan saved a company ~1m USD/year with latest commit; and awards him $100k. And you see this unusual bonus, how are you going to know 99% of that doesn't go to someone other than Ivan?
Edit: there are many ways to hide bribes, of course. Typically it involves overpaying for something. It would be hard for even a non-corrupt tax authority to tell the difference between having bad negotiation skills and bribes.