While the White Council may not know that Harry has the Spear of Destiny, they know he's the Warden of Demonreach (They can't be in denial about that any longer.) and he had the power to bind and imprison Ethniu from the shores of Lake Michigan. Harry didn't have to lure the Titan to the island and the Council leadership would have no way to know that Harry was operating at the limit of the Island's power. On top of that, Harry pretty much told Carlos that he not only had Ethniu, but that he could control her to do his bidding. And he explicitly said, "That's a threat."
There might be several other magical artifacts that would have allowed Harry to do what he did, we can't be certain. What the Senior Council can be certain of is that Harry has something, very powerful, that allowed him to get the job done. On a related subject I wonder about the placard that Harry convinced Mac to hang up in his bar. Is it still there or did Mac take it down afterwards? The reason I wonder about this is all it would take to give the Senior Council a big clue is for Ramirez to have stopped at Mac's for a beer before he left town. (Maybe because he wanted a beer and maybe to ask some questions.) My guess is that a wizard would be able to detect the power coming off of that thing, even if they couldn't identify it. It's likely a description of that item would be enough to make someone on the Senior Council realize what it is and figure out what else Harry might have in his possession.
Finally, remember that in Turncoat, the Gatekeeper found it disturbing that the "little folk" were organizing, and that was probably based on nothing more than rumors that couldn't be substantiated. It was one of those unnatural occurrences, similar to the sky raining toads that told the Gatekeeper that something extraordinary and dangerous was occurring. Finally learning that Harry was the reason for this organization and that he controlled their purpose must have really freaked the older wizards out, because if a mortal could control an army of small fae, that could mean he could control an army of large fae. Oh wait, Harry did command an army of large fae during the battle. I think it would have been hard for Cristos, Ebenezer and LTW not notice that at the vanguard of Harry's mortal forces was a screen of some really nasty members of the Winter Court. So, their worst fears were confirmed.
