We turn down candidates all the time. No doubt that many of those go on to find success elsewhere, especially the ones that we turn down for fit (cultural or candidate expectations).

It's an open question whether a negative for fit is a false negative or not. If someone wants to be a foo and the need that I have is for a bar, it's a true negative for us, but when that persons goes and finds success as a foo somewhere else, I'm not sure where to put them your data set.

Even for the false negative, how would I possibly know what career path they took after that? I certainly hope it turned out well for all of them, but I have no practical way, not to mention no incentive, to find out.