I have a theory. but it is less paranoia like the others.
Someone a stuff member (who knew the location of all working camera, who was able to open an alarmed door to the roof, and who knew that the girl has some weak points – that you can tell after a few personal meetings (like reception desk talks)), invited her to the roof, for some activity (and this person is holding the button of the elevator from the hallway, not letting the girl
leaving, till she gave up, and steps out and leave with the person (human not a
ghost, only a human can be so evil). and then something went wrong up there…