Same as previously, once the gate is open, fly inside the emergency shaft [#1], leading to the damaged room with the massacred janitor [#2].
Take it and return up but this time, instead of throwing it to the cauldron, fly upper [#4] and throw it on the plate. When you pull the piston, a camera will show up. It will scan the janitor and massive gate will be open.
Press the button to open the manhole and then fly into the tunnel. Once you get to the other side, fly up heading to the exit. Congratulations, you got out of the underground!