Susan can be found near one of the third-floor storerooms. She's been coping with returning to her body - which is, once again, shorter than she expected it to be, with narrower shoulders than both her brief stint as Thomas and her first adulthood alike - by trying, with almost no avail, to bully the Mansion into giving her anything else from Narnia. (She's perhaps even grumpier than might be expected, following a bodyswap, because she's been working very hard on respecting Lancelot's evident need for space instead of crowding herself against him the way that she really wants to be doing.)

So intent is she on yanking the door open, peering inside, and closing it again with a huff that she doesn't notice Lancelot approaching.