Usually things like that are explained later, or left never explained. I guess I have gotten use to that with the show. But really, the dialogue was not made out of cardboard as it usually is on one side or the other (of course I could just be getting use to bad acting, seems to be everywhere these days).

I think the whole Juliet and the key thing was to establish an undisputable distrust of her on Kate's part and the part of the viewers. They didn't do the best of explaing that what happened, but I think that is what they were getting at.