On the roof toward the batto you will find a railing and beyond a manhole, stand in front of the railing where the manhole is and press, and a scene will start
Once there you have to click right in front of the railing and the animation starts, if you are already underneath it talk to Sline and go around the area you will find a stone, click there.
I specify that this scene is a scene from 0.5, if you are playing an earlier version it will obviously not be activated