Her smile widens about 4 times, enough to make the Cheshire cat look sad! There's no implication that she stopped smiling. Obviously she wants something, so maybe something you say catches her for a moment as she realises she might not get it which would reset the smile. Also, if someone was smiling that much and creeping you out in the moment, would you remain? Feels a little unlikely, so maybe it's a bit more of a normal interaction and that you look back when you leave and catch her smiling creepily and she has to glance away. Maybe it's just that the image of her never smiles too.