Of course William knew that, the prophet doing his best to look neutral on the matter and failing. He's too busy looking impressed to be able to put his usual mask on, and if Gabriel looks hard enough he'll see a slight hint of uncertainty. Why is Gabriel showing him this? Is it a trick? The mirror isn't sure, and it's hard to believe it's an legitimate thing when he lives in a world where it's every man for himself. Obviously the other shoe will drop at any moment now.

"I might have known that bit."