In Blood Rites Bob drops a pretty throwaway hint - "a fine thrall is so controlled they might not even know that they are a thrall at all, and it last long-term." He says this when explaining the difference between thralls and Renfields. Harry responds "just like Dumorne did to Elaine" to which Bob says "Sure - but it takes a lot of time and a certain amount of empathy"
So we know that Elaine was enthralled by Justin Dumorne - this was well established in Summer Knight (at least by Elaine's word...although it makes sense as Justin tried to do it to Harry and Elaine likely wouldn't have just gone along otherwise).
But two things are implied here:
1) Justin had a certain amount of empathy towards Elaine and Harry.
2) Someone is probably a thrall in the series, but they have no idea they are, and it has been going a long time.
It makes sense that Justin had a certain amount of empathy towards Harry and Elaine. From his scenes with Harry (aside from combat) it seems that he was a tough love sort, but did show warmth and affection. He may have been mad or corrupt, but he seems to have actually liked if not cared about his foster children.
Secondly, if Justin were Cowl - Elaine still being a thrall makes perfect sense. Hell, even if he isn't it still makes sense. She has definitely acted quite suspiciously throughout the series, with only plausible explanations helping her get out of trouble.
But what if Harry was the one still enthralled?
Would he even know? Would someone have picked up on it? There are some hints that this might be possible. For instance, in Summer Knight the Gatekeeper says Ancient Mai fears that Justin's teaching may have warped Harry in ways he cannot even see. While it is meant to read that it is more to do with the values that Harry might have learned from his time with (and even against) Justin, it also hints that Harry might not even be even able to tell if he has been shaped and controlled so. Another thing is that beings are constantly messing with or trying to mess with Harry's mind and Will. It is one of his greatest fears and weaknesses, and is a recurring theme in the series. Lasciel, Mab, Molly have all successfully messed with Harry's mind without him knowing for a long time. It could well be that a gifted psychomancer like Justin did just that, perhaps Harry only believes that he beat Justin and that there was a fire or perhaps there was a fire and a duel but the enthrallment wasn't fully broken (as we do know that Justin was attempting to enthrall Harry just before the final duel, and indeed got Harry to drink human blood as part of it and we all know how powerful human blood is, magically speaking). Constantly, most of the Council (especially the Senior Council) are worried about him being this Darth Vader like villain. Many of his enemies often believe that he will join their side, and knew his Mother (which is part of the reason many on the Council worry about him). Hell at one stage Nicodemus even believes that Lash has taken over Dresden and that it is in control. All this could be foreshadowing.
The obvious flaws are that during his star trial he would have likely been examined for such tampering, and Ebenezar would have looked too, plus likely a member of the Senior Council (unless it was Simon who vouched...which might hint all sorts of things, we know he is implied to have misled the council many times before). But presumably Mab or Molly would have noticed when they poked around in his head, or anyone else too.
Still though, the enthralled sleeper agent is a distinct possibility based on Bob's statement. I think either Elaine or Harry, or both, are enthalled.
Thoughts?