In the last episode, the woman that killed the guy in farm says:

I had a feeling, someone was there with us...

And at that moment, we hear Red John breathe. But not only breathe, but take two steps as well.

I want you to open the episode, and listen to it.

... Breathing starts...
... RJ takes a step...
... Sound of something being dragged on the ground...
... RJ takes a step...
... Sound of something being dragged on the ground...

At this point, Red John is hiding. He is relaxed that he is not caught. So there is no way that he is carrying something heavy by dragging it on the ground.

I think it is his leg. He must have a problem with his leg then.

It might be a clue, or it might be something else than his leg. This can help us someday, I just want you to keep this in mind.