Let's think about who would have wanted to bug the office, I think it's very important.

1. Bertram is out, he could have bugged it himself anytime.
2. Haffner is out, he was in the office a couple of times in that episode, could have done it himself.
3. Reede Smith: I refuse to believe this nobody is Red John, if he is, this show will have the lamest         ending in the history of TV shows. He is a random stranger who nobody's ever heard of before season   five.
4. Stiles could have done it, but let's be honest..He is almost 80, he is dying, the FBI is looking for   him...It just doesn't look like it's going to be him.
5. Sheriff: He could have done it.

What we learn from this is: Whoever Red John is, the other tigers don't know. Because if the sheriff is Red John and the others know, he could have asked Bertram to bug the office, no need to bring in a silly private detective. (same for Smith)

So I think it's either the sheriff, or someone we think is dead or not on the list.
It cannot be Bertram or Haffner, they could have bugged the office themselves. For me it also can't be Stiles and Smith for the reasons I mentioned above.

It's the sheriff, or someone "dead" or a person not from the list.