On the Dictaphone Sophie said the man was a good whistler. When she says this they show Sheriff McAllistor whistling. However if you look back at the scene where Haffner goes to see Lisbon at the hospital, in addition to the creepy feeling and the dream, Haffner exits the room whistling. Whistling!

This is just my theory. Personally i think its Haffner. Add up with the whole visualise and him being in the private sector plus now the whistling makes him RJ for me