Claire's "friend" Locke/smoke monster stops by

Back at Claire's shack, Jin lies to her and says that Kate doesn't actually have Aaron. He tells her that he saw Aaron at the temple and that he'll help her get him back. Claire is relieved, because she says if Kate really did have Aaron, that she'd have to kill her.

Locke suddenly pops his head in the shack and Jin is stunned and says "John?" Claire tells him that it's not Locke, it's her friend. Locke/Smokey gives a fiendish smile and that's the end.

So how does Claire know that Locke is not really Locke? Will we get to see what happened to her during her three years on the island? What exactly has Locke been telling her besides the lie that the others have Aaron?