Escape - Prison Escape
During the escape, you'll reach the rooftop at night in the rain and you'll break into the Warden's Office. Once inside, as either character, walk straight ahead onto the balcony to notice a caged bird, and let it free. The achievement unlocks for both players.
This achievement can be seen in the video guide at 1:22:NOTE:
Achievements are only available to owners of the game. Players using the "Free Trial" friend pass will NOT unlock achievements.