I had thought it was Kirkland, but he is out after this week so I looked
a little more and now I am pretty sure that it is McAllister.
It
has been stated several times over the years that RJ has been on the
show since the beginning, but would be hard to find. McAllister appears
in 1.02 and is not seen again until this year. Partidge was 1.01 and was
thrown out there as a very obvious suspect, but tucking the real RJ
into the much more forgettable second episode as a potentially creepy,
but otherwise forgettable small town sheriff would keep him well hidden,
still there all along. It should also be noted that McAllister looks
very much middle aged in his first appearance, which is the basis for
many people eliminating him as a suspect based on his current appearance
(maybe another diversion?).
The clues are subtle, but every time he
has appeared on an episode, he sneaks up on someone very effectively. In
RJ's appearance (2.23) he seems to leave, but sneaks around PJ for one
last word. RJ also sneaks behind Lisbon in 6.01. If the theory is right,
the birds will tie into it somehow.