My guess? CBI Ron. Why?
In the last episode (season 5 ep 7) in the end the Bob guy says to Patrick Jane something like, you dont know me but i know you!, then when they go away, right before the episode ends, who is on focus? yeah... CBI Ron.
CBI Ron is always appearing. Always near Patrick..
And please check episode season 1 episode 16.. When Grace Van Pelt's new boyfriend is chasing van pelt and patrick jane in the parking lot with a gun, theres someone that runs away in his car. Who is it? CBI Ron!
A CBI Agent that watches some criminal with a gun trying to kill two CBI workers, and simply goes away without doing anything? That's really suspiscious for me.. I mean, he could not be Red John, but in some way he must be an accomplice or something like that. He is not a good person.. And we never heard his voice I guess..
Because why would the director of the series put a secundary actor like CBI Ron so many times during the 5 seasons?
Rate my theory guys :D