The comment makes Will wonder when Hannibal had uncovered Will's deception, where the fork in the road came in, because it had all felt seamless to Will until tonight. Hannibal's suggestion they leave without carrying through with their plan to murder Jack had been a line cast out to see how Will would react, if he'd take the bait or swim away uninterested.

It reminds Will of the metaphor he and Jack had used on the other end of this path they're on, about how to make a fish bite, even when he's not hungry. It makes Will wonder if Hannibal had any tactics he'd planned to use if Will hadn't bitten, something to entice him into doing what Hannibal wanted, to make Hannibal's lure be the one thing he wanted, despite everything he knew. But none of that, of course, had been needed. It turned out that Will was hungry after all.

"You knew, before I told you, that I'd been working for Jack these last few weeks," Will says, and it's not a question because they both know it's true. "How did you know?" Will asks, turning his head just enough that his eyes can move over the side of Hannibal's face, try to see his expression. It's just to sate Will's curiosity, to see where he'd misstepped, what accidental evidence he'd let Hannibal find.