"There is a clue buried in that final sequence that will pay off big time in the finale of the Red John story. A very big clue. No one will get it from the clue as it is laid out in the episode, but it will pay off later." - Bruno Heller
My guess: Is that Red John is left handed, something that's been suggested in previous episodes; after he ran out of the house Dumar and Jane were in, his tea cup handle was shown pointing to the left. Later, Rosalind Harker calls Jane and Roy is shown holding a tea cup with his hand.
I've checked for left handed characters in all previous episode.
There were a few possible ones, but I'll stick to the remaining suspects:
Stiles, Haffner, Kirkland, Bertram are all right handed.
With Smith and McAllister and it's not certain, and both of them are displayed as "in transit" on the cell tracking monitor.