Follow the janitor

Get out of your car and enter stealth mode. Follow Harvey as he goes on up the stairs and toward his apartment. Keep a good distance to avoid being noticed. Head on up the stairs for the apartment complex and go to the right. Keep far enough back to see what he is doing but not so close that you draw attention to yourself. Let him enter his apartment and then follow him in.

Once inside the apartment grab the bag. This will trigger a cutscene between Michael and Harvey. Harvey seems very willing to go along with it.