My brother, as a manager of a big team in Belgium, had once to face a complain to the HR about he harassing a secretary. The reaction of the company was immediate, and in my opinion the right one. They contracted a specialize company to investigate, collect the facts and present their conclusions. The conclusions would be sent at the same time to the alleged victim, the HR and the manager. In this case the conclusion…
I think that is a good start, but we should be aware of the flaws in an approach like this. I had friends at a company with serious security breach. There was a hullabaloo, all sort of important people in the company upset and questioning whether they should fire the head of tech. To calm the waters, the head of finance hired an outside firm to investigate. All good, right? Here, the head of finance liked the head of…
Independent investigation is great, but you're right to observe that "outside" is not necessarily "independent". I suppose there are a few solutions to that: you could break the financial link (in the bond case, get consumers or the government to pay raters), or you could establish a culture where inaccurate results destroy reputation.
I'm sure there are auditors who do objective work for the people paying them simply because they value being known as reliable (and who are hired because the auditee wants a result people will trust). Without some precedent, we probably shouldn't assume that rosy case is happening.