You can only find this free battle star after unlocking the Quiet on the Set Loading Screen.
This is the Quiet on the Set Loading Screen for reference:
If you look at the picture carefully you can see a battlestar on one of the Prison's towers.
If you carefully analyze the setting and the location you can figure out that this is the Northeast tower.
See screenshots and maps below to help find the location of the secret tier..
- You can only locate the secret star after you have unlocked the Quiet on the Set.
- There is a bug where sometimes the battle star does not show up. If that happens try logging out, then choosing the Quiet on the Set in the Locker as the active load screen, and then trying again to go to the secret spot.