There are 2 Chimneys that vent the smoke out of the catacombs below La Cort de Miracles. You want to find and block both of these. This will fill the catacombs with smoke while you are escaping and make it vastly easier to get out without being seen.
Head straight forward and down from the starting point. You will hear some people talking about a spot being hot. You want to go over to them. You will see the chimney with a few people around it. Head over to it and interact with it to block it off.
The second chimney is none too far away. Head over to the West of the Starting point. You will see it nearby a stationary Extremist.