I’d be very surprised if the FBI comes up with anything we don’t already know. Most of the allegations have been submitted in writing to the judiciary committee. And Kavanaugh, Judge, PJ, etc have all submitted letters refuting the allegations. You’ve have the media, which in many cases can get more information than the FBI through anonymity.
I saw one source that said the FBI did have access to phone records going back to 1982, but not sure if that is really true or not.
Personally just not very worried. FBI has already done this 6 times, and they do go back and talk to old neighbors and HS friends on background checks.
The point is to make childish behavior in High School keep a judge off the Supreme Court. Once you have established that, finding childish behavior is easy.