I think it's CBI RON
I mainly base this accusation according to 2 points:
1-The best way to hide something sometime is to put it in plain sight. so you look over and over around it but you can't see it because it is not focused on but if you get yourself out of the picture and view it from other angle you will see it.
2-He is there almost every episode like waldo but you have to look for him and watch his subtle moves and behavior with other people especially red john disciples.
n.b., the internet sources says he appear on the series starting in episode 17 of second season "the red box" and this is wrong, they started credit him since this episode but he was first seen in episode 7 of the first season "SEEING RED" and many following episodes but was not credited.
