There's Rupert Neve's famous story about Geoff Emerick apparently being able to hear artifacts around the 50K region. Nice story, but check the rest of the story - GE was pissed off that Neve were sending various oiks to him who hadn't a clue about how to fix problems. I could quite believe that someone who's paid a fortune for a top-line desk and was getting shafted on support might invent an issue just as a way to get someone on site who actually knew what they were doing. Kind of like the Van Halen brown M&Ms - the important thing is not the minutiae of it, but just that the other person's taking their responsibilities seriously. So the fact that Rupert Neve happened to find faults on two channels might be purely coincidental, when the real reason for GE being happy could be that it was RN who was personally doing the work.
So ignoring that, what's the *documented* highest frequency that anyone's been proven to hear in proper aural testing?