I've seen every single episode of the mentalist, and I remember reading somewhere that that Bruno Heller said that Red John would almost be a dissapointment, because it's not someone super secret; this is why i think Red John is Rob Kirkland, and for a number of reasons. 

First of all, we know RJ pretty much has the skillset of Jane, in terms of both hypnosis and intelligence; just used for a much different cause. 
Furthermore, a couple episodes back when Lisbon asks him(Rob) if he was on that visualize farm where RJ was known to have been years ago, he doesn't answer. 
At the end of Series 3, The guy who Jane killed, believing it was RJ, said he was going to move on and create a new identity, or something of the sort. 
Now, from last nights episode, the conversation is pretty much self explanatory, however what i find interesting is when he taps Jane on the arm walking away, and Jane's face looks almost as if he's remembered something. How many times have we seen Jane tap someone in a certain place for a trigger to come out of hypnosis. 
My theory is this - Rob knew jane on the Carnival years ago, and something went on between them; at some point in the future, he hypnotised Jane to forget his identity. He did the same to Jason, and that's why he wanted to know if he remembered him, to see if Jane could suspect him and could have broken this hypnosis.
That Lorelei was found in a clown/carnival environment is also significant.



Hey, i'm sure after the next episode all these theories will look crazy, but that's just my absurd theory right now. Likelihood is that the clues have been hidden throughout all 5 series' rather than the big sudden reveal.