Harvey asks the bank officer for privacy in the vault, then when she is alone she closes her new box and uses another key she brought with her to open a box that doesn't belong to her. In real life, that would not be possible -- either the boxes would have two keyholes, one of which has to be unlocked by the officer, and/or she would have been given her requested privacy with her own box in a separate room away from the other boxes. (Plus there would be other, lesser-known safeguards against inappropriate access in a vault containing a box owned by the President of the USA.)