I know, I know. Partridge is already dead. Up until about one minute ago I totally agreed with that. However something got me thinking about this again and here is what it is.....

Remember when PJ & Lisbon were shown the picture Cho took of Partridge in the morgue? Lisbon asks PJ what he thinks it means. PJ replies with a question, What do you think it means? Or something close to that. We, the audience were, all focused on the skin being removed from the shoulder. What if PJ noticed something completely different? Maybe a birth mark, or the color of the skin or something specific he knows about Partridge that didn't appear in the photo. PJ acted very decisively after seeing the photo. I think that was the moment he figured out who Red John is. Partridge. Since he wants RJ for himself he wouldn't have pointed this out and had the body be more closely examined.