I have a feeling that you are at the top floor. That is the only level I know that has a door to the outside.
Go back inside, you should be on a platform with 2 instrument on either side and the elevator at the center. Go to one side, then walk the rail to the center of the room, turn around and then see the elevator.
If you think you're at the disposal place, is there a round stand at the center of the room? If so, the button will be on the (gee, can't remember) right side I think of the elevator structure frame.