I think I figured out who RJ is. Seriously, this theory is based on all the clues given in the 6 seasons until now and lives few if any holes in the script.
Are you ready for the revelation?
RJ is an extraterrestrial being with shapeshifting abilities. He kidnaps people, give them some drugs that cause amnesia for some days, then takes their shape and replaces them in the real world. After he is done messing around with Patrick, he retreats to base and resends the original person back. In this way, RJ is in the list (appearing at defferent times as any of the suspects) and in the same time he is not on the list (appearing as the policeman who first arrives at the scene to hide evidence etc). Sorry to ruin it for ya guys, but I had to tell you.

Everyone can be RJ!