Some observations (I have not firm suspect albeit Bertram is my favorite, with Partridge as a far out dark horse):

1) It looks like Stiles had a tattoo removed from his shoulder. That could mean that he has been a member of the "three dot" /Tyger conspiracy, but isn't any longer.

2) Also, I'm not sure that the shot heard is from PJ's shotgun. It could have been from a pistol/handgun. Furthermore, it doesn't make sence for PJ to start shooting without any further questioning.

3) The voice saying "Wait, no" sounds like Bertram to me, but I'm not sure.

4) IMDB lists Bertram as being in the next episode (S6E7). However, that could be a flashback, although I personally don't think so (I have no evidence to support this). http://thementalist.wikia.com lists Smith in addition.