Plus Eb. Harry shows up to look for someone keeping an eye on Justine. He spends an hour and a half looking for magical surveillance, finds nothing, and then has Eb decloak at his rear and call him an idiot for not spotting him.
Eb has a plausible explanation for why he's there, but he's also perfectly capable of hiding himself from Harry (as he did) and if it had been any other wizard veiled near Justine's place he would have jumped to the top of the list.
Alternative theory: The White Council was spying on Justine, which means Martha Liberty's spy network and scrying spells way, way too subtle for Harry to spot them. And it probably means Liberty alerted Eb to his former apprentice skulking around at Justine's place.
No he doesn't. We know Justine's place is a good place to look for Harry because we've read all the books. Ebenezer shouldn't have a clue about Justine or where she lives, unless. Think through the books and how Eb has learned about Thomas.
Ebenezer probably knows nothing about Thomas being at Bianca's party or that Thomas helped Harry after Duke Ortega tried to cheat during the duel at Wrigley Field. But Ebenezer hears rumors that Harry has been seen in the company of a White Court vampire. Harry directly intervenes in White Court vampire politics, which seems to help Lord Raith. We can't know if the White Council has figured out that Lara Raith is really in charge and that her dad is now just her puppet, though at some point they probably figure it out. From Captain Luccio, if not before, Eb learns the White Court Vampire Harry is seen hanging out with is Thomas Raith. From Listens to Winds, Ebenezer finds out Harry fought a naagloshii in order to return Thomas to his people. Finally, Ebenezer sees Thomas fighting alongside Harry and his other allies at Chichen Itza. After Harry seemingly returns from the dead, Eb might have heard that Harry was again seen with Thomas.
In the past eight or nine years Ebenezer has visited Harry in Chicago twice. The first time Eb visited to help hunt down Mavra and the second time Eb and Harry had dinner after... I don't even remember which book any longer. (Small Favor or Turn Coat?) It's unlikely that Ebenezer knew about Thomas the first time because he would have said something about Thomas, especially after Eb told Harry that Lord Raith had killed Harry's mother. I think it's also unlikely Ebenezer said anything during their off camera dinner together. Harry would have said that Ebenezer was repeating himself, that Eb had said this the last time they had dinner together, but Harry didn't. So all of this means that Ebenezer McCoy has been doing some deep research on Thomas Raith, or having someone else do it for him.
Also, Harry would have spotted any wizard hiding under a veil. Harry spotted Mavra at Bianca's when he was much younger, and you have to assume that Mavra is probably one of the best there is at veils because her continued existence depends on it. So that means Ebenezer was likely watching Harry doing his search the entire time and only revealed himself when he realized Harry was about to use his Sight and would spot him. Why? I mean why didn't Ebenezer reveal himself earlier, unless he didn't want Harry to know he was there at all.
Now I want to point out something no one else seems to have mentioned. Harry asked himself why Thomas attacked Etri. (Assuming it was Thomas and not something complicated; like Austri was mind whammied into attacking his boss and Thomas was actually defending Etri by killing Austri. Normally Occam's Razor would rule something like this out, but at least half the damn book is missing so who knows what new clues will turn up in the first few chapters of Battle Ground.) Evanna said, "Well Raith? Do you have anything to say?" OK, she could be acting, but at least she's acting like she and her people have been wronged. Justine asked "Why? Why Harry? I don't understand why he would do that." Lara said, "Thomas couldn't have screwed this up any harder if he'd had a year to plan it." That statement is a bit vague, but what does Lara stand to gain by attacking Etri? Again, maybe we'll find out something in Battle Ground, but right now it doesn't make sense. You know who doesn't ask "Why Thomas?" or anything similar? Ebenezer and the wardens. It's almost as if they don't need to ask any questions because they already know, or at least one of them does.
Finally, Harry says that Carlos and the Council will, "...want to meet about the ramifications of an apparent assassination attempt by the White Court on Etri..." Except that doesn't happen. Ebenezer flat out tells Harry that Thomas is going down for the killing. There's no political discussion or what it means for the talks. Carlos and the other wardens think Harry has been turned by Lara so at least their behavior makes some sense.
So what I'm left with is that either Ebenzer is responsible for Thomas' action or he knows who is; maybe Cat Sith, and has decided this is a great way to get rid of Thomas.