I've gone through all of the Red John related episodes since the first season and the most profound hint that I've found so far is in episode 1x11 Red John's Friends. About half way through the episode, Red John hacks the medical records for the CBI and has a short chat with Jane. Pause the episode when it goes to Red John typing at 22:31 on the DVD's and you can see the younger reflection of Bob Kirkland without any facial hair. Go check it out right now I kid you not. Bob Kirkland is RED JOHN.