It's a pretty W-A WAG, frankly... particularly when Occam is patiently standing there with an obvious alternative. We have WoJ that they're "frozen in carbonite." We have (as @morriswalters points out) Lasciel's Sigil being gone; not faint or obscure, but gone.
But Bonnie wasn't, was she? Note in Skin Game Lasciel referred to her as
her child, not Lash's child. Though I agree that the coin should have been frozen in carbonite, two things, 1] if he didn't fear any connection to the coin at the end of White Night, why not just leave it there? It was unlikely anyone was going to come for it. Maybe Anduriel knew, but then he must have also known that the Sword of Faith was in plain sight in Harry's umbrella stand for years, yet no one tried to steal it to destroy it. 2] Is Bonnie a connection between Harry and Lasciel?
But consider: Nic had given Harry the coin. That paints a great big "Denarian Coin HERE" target on Dresden's dingy little hole (if Nic ever gets the report that the sigil is gone from Harry's hand, etc)... and on the home of Mrs. Spunkelcreif & the Willoughbys.
Actually Nic knew that as of Small Favor. Also Michael seems to think just because Harry got rid of the coin, that it wasn't over..
page310
Harry has told Michael there is no more shadow, he'd given the coin to Father Forthill, Michael has doubts.. [Since this conversation happened in Michael's pick up, Andruiel might know as well.]
"How do you know?" I asked.
"Because in two thousand years,no one has rid themselves of the shadow of one of the Fallen--except by accepting the demon into them entirely, taking up the coin, and living to feel remorse and discarding it. And you claim you never took up the coin."
"That's right,"I said.
"Then either the shadow is still there," Michael said, "still twisting your thoughts. Still whispering to you. Or you are lying to me about taking up the coin. Those are the only options."
Now we believe that Michael was mistaken, and Harry is the exception to that rule. But then again, if Bonnie is the connection back to the coin, then Lasciel still could have made a showing in the office in Changes.
Later on during the fight on the island, Nic asks the shadow of Lasciel to disable Harry so he can
help himself to the Sword of Faith. Harry tells him on page 382
" Lasciel's shadow," I told him, "doesn't live her anymore. The Fallen have no power over me and neither do you."
So between what he told Michael in the pickup assuming Andruiel was listening, and what Harry told
him, he knew the coin was no longer there. Yeah, it is possible that Andruiel wasn't listening in on Michael and Harry's conversation.. But then again if what Michael said about the coin and the shadow is true, then neither Nic nor Andruiel would have believed the influence was totally gone.