There are kids playing downstairs when you enter the building. The little girl says,"Don't talk to me.... if you do, I can't hear the elevator..." Its like she knew you were gonna go upstairs if she told you that. I'm not sure who the ghost girl is. Similar things have happened in the Darkrai Event and the ghost girl in B/W and B2/W2.
Probably the more you progress in the game, the more you'll learn about her because she says, "No.... You're not the one..." So just keep progressing in the game and you'll find your anwser.