The very first and most important clue about Red John is that he either has a person in CBI or he is there by himself. In my opinion it's him because remember in the episode where Bosco died his lover was smiling at an approaching figure. However the cops around her ignored the figure which means that he usually is there or is not as important to be acknowledged. The woman also said that she is curious why Jane didn't become friends with him even though they had shaken hands. It means that RJ must be somewhere in Janes everyday environment. Another point is that RJ broke in to the computer software of CBI before. But what if he simply had an access to it? And who has access to most of the facilities in places like CBI? Access to the room in which friend of RJ had been burned? It has to be a man so we can exclude few possibilities (both of his lovers talked about a man) and an important figure like La Roche is too visible.
In my opinion Red John is possibly one of the caretakers.
