Who was that girl leaving Noahs apartment and why did Rachel hide from her?
While sometimes mistaken for a suddenly fully grown Samara, this person is in fact Beth, Noah’s assistant (the one who kissed him on the cheek in an earlier scene). And she was going up to his apartment, not leaving it. As for why Rachel hid from her, there are a number of different theories ranging from shock to spite to sheer inability to act.