CAT!
Red John has a cat.
In one episode, possibly in S2, or maybe in S3, RJ calls Patrick Jane. During the small chat, a very obvious "Miaoow" sound comes.
A very important call, and we hear sound of a cat. It is impossible to be coincidence, it is included on purpose.
Who had noticed it? Can you make any connections? Who has cats in series? ( It was not the very fat guy, animal supporter, who was falsely arrested for killing people doing anti-animal rights lab tests, I had checked it, but you can double confirm if you have time)
