Open the hidden door in the Garden after you get all the 4 notes. in the tunnels go north and west and you'll find a key. When going east you find a girl, not sure if you need to pay here a visit. After that go back to the first building and open it with the key.
In that building I knocked on the third door on the right, not sure if that is needed to progress the story. A bit further in the hallway, 2 doors after the note on the right wall, you find the keycard in the room on your left.