The obvious choice i would say is Kirkland.
We know in one episode he sent some guys to break into PJs "thinking room",
PJ noticed that someone had broken in there so I think thats why he's not that spooked about RJ knowing about his list. 
Kirkland also killed a person with RJ connections before PJ had a chance to speak with him.
And he was eavesdropping when PJ spoke to Bertram.
All we know is that Bertram and Kirkland got some "fishy" going on.
I would not be surprised if it turned out to be one of them, or both. 

My alternative option is...
PJs father! Alex Jane
Whatever happened to him? Did he die? Where did he go?
He is the only one who could know about PJs happy childhood memory. 

And one more thing....Why oh why must they make cops so mtf stupid!? Why walk in to that house all alone without backup or making van pelt checking the GPSs during?? Then case would be solved!
Risky move by RJ i would say.
But what have we learned from tv-shows?
That cops are god damn stupid