It was just like Carrie that she would spot such a young woman, though a passing glance would cause one to see more ghost than woman. It could've just been that Carrie was in a way the same, easy to ignore and seemingly moving in a time frame different than our own.
Maybe it was the sense that the young woman 'stood out' and wasn't normal like Carrie, that the young woman on the roof waved a hand at her to get her attention.