"I'm having the strangest sensation," she whispered.
No—was it already too late?
Her eyes narrowed into the shape in his sketch and she came back to him, her lips parted expectantly. "Tell me I'm mad, but I swear I've been right here before…"
So it was too late. He looked up, shivering, and could feel the dark descending. (Prologue.35-37)
From this interaction, it's clear that this is a tipping point for Daniel. Every time Luce says this to him, it must put his guard up, because it's a signal that she might disappear at any moment. That really puts a damper on an otherwise uneventful evening, doesn't it?