Look at the promo video of Season 6, on this link :
http://www.melty.fr/the-mentalist-saison-6-mariage-en-vue-et-nouvelle-video-promo-video-a212804.html

Now, stop the video at precisely 00:20 ! You will see Lisbon and Jane standing in the bureau of the CBI, with a smiley on the window and one on the floor, and two corpses : the ones of Grace and Rigsby ! What sentence is written in red ? "Behind the Smiley"
Later in the video (at 00:25), we can hear the voice of Jane saying loudly "LISBON !", and her being surprised. What can you see behind her ? The one smiley on the window that was before in the scene with the two dead bodies !

Now, what can we suppose ?
First : Both Lisbon and Jane discovered the two dead bodies of their team in the night (the flashlight of Lisbon), Grace and Rigsby were dead a few minutes ago.

Secondly and THE MOST IMPORTANT ONE : Red John didn't fear coming to the CBI at night to kill two experimented agents in the same place. That means :
- He knew he wouldn't be suspected if he were found in the CBI before escaping. It IS someone that can regularly move in the CBI ;
- He knew he would be able to kill them both before they could react with their guns. Grace and Rigsby didn't have the time to react when Red John killed the first one of the two. It must be someone they knew from the CBI, not someone like Sheriff McAllister who would be directly suspected if seen in the middle of the night in the office.

OR : Red John managed to hypnotize and kill them with them reacting. We know Grace likes Bret Stiles's speech, can't resist to his words. He could have easily hypnotized her, then kill Rigsby right after that.

Who could be seen in the CBI late in the night without being suspected ? Only Gale Bertram could.

So, I'm left with only two suspects if Bruno Heller wants to make things coherent :
- Gale Betram, being logically there because he's the head of the CBI ;
- Bret Stiles, possibly because he would easily hypnotize them and is the only person we know that can be anywhere with some excuse.

I just put Gale Bertram because I can't choose the two, but I'm still thinking about it. I have looked for tips in the 6x01 end for both of them. Have reaaally interesting things, but can't decide whether it makes sure it is one or the other.

I'll quote one sentence of a pretty clever guy we all know : "I'm getting close, I'm getting really close ..." And you ?

Bruno Heller said the clue was "buried" and fans wouldn't understand it until later on down the line. Therefore, I'm not going to bother thinking about it because bit wouldn't be anything we expect anyway. Just a thought...

It's not Partridge, since he is dead.
I think the phone tracking maps might be the clue in the final scenes. Since there were no other phones showing up co-located with Partridge's, we can assume RJ left his phone somewhere when he went to kill Partridge. Which is also why he used Lisbon's phone to call Jane. That makes sense, as he knew about the tracking. So, if his phone was left somewhere, it likely isn't "in transit." According to the map Lisbon was looking at, that should eliminate Stiles, McAllister, and Haffner, who were all in transit during the murder. So that leaves Kirkland, Smith, and Bertram. Smith and Bertram don't fit the physical profile of RJ, which we know from the few times he has been seen, and also from the description by his blind girlfriend. That leaves Kirkland, who left his phone in LA when he realized he was being tracked, and went to Sac. to kill Partridge.

I think he killed Partridge because Bertram thinks Kirkland is on the up and up, and is working to find Red John. Bertram must suspect Haffner, who was inside the CBI. Which explains why he and Kirkland discussed whether they can trust Lisbon. Would she blab something to Haffner, accidentally or on purpose, that would compromise what Bertram thought he and Kirkland were doing? Partridge's death will validate Haffner's guilt to Bertram.

But who really knows? RJ could have purposely left his phone in a taxi, so that it would appear to be "not left behind." And there are still a lot of things that don't make sense. Where WAS RJ hiding when he popped out and grabbed Lisbon? That suggests it had to be Partridge, "hiding in plain sight." And is it true Jane didn't meet Kirkland until AFTER he was told about the hand shake clue? That suggests it ISN'T Kirkland. I hope it all makes sense after all is said and done.

Red john is janes stepbrother. after he left his father his father had another child in the hope it would be like jane but after years of being compared to his brother he also left . The smilie face is his clown makeup from the carnival where he and Jane worked but while Jane was a headline he was the clown . That's why he knows about janes past and is obsessed by him . Then years later he is publicly ridiculed by Jane the brother he was always told he wasn't as good as . He then kills janes family and starts a game with him. He has cosmetic surgery to change his appearance which is from the episode where bob asked a known red john accomplice if he recognises him then is relieved and kills him .