Here's what I think happened in the 2nd episode of season six:

First Theresa's dream had lots of dark imagery, but who actually attacked her? Haffner!

Next, during Patrick's visit, the doctor came in looking for Carmen Lee.  I think Haffner  just wanted to know if Theresa was alone, and send the doctor in to check for him.  She waited around in the hall until after Patrick left, you can see her when he is on the phone.

Once he knew she was alone, Haffner came in to see if WHAT Theresa remembered, to see if she knew it was him.  He sensed she was nervous, but I don't think he believed she knew he was Red John.

It struck me how good his posture was compared to the other suspects, which was one of Sophie's clues.  He is also the correct age and body type, if the clues are to be believed.

It gave me shivers when he said that one day they would have "a time" in the future.  That doesn't sound like someone who is planning to get to know you for any length of time.

And the whistle, well, if you go read the lyrics to "Jimmy Crack Corn"  AKA Blue Tail Fly, they are about a slave getting away with murder, and are offensive by the way.