Escaping the Coldrigde Prison non-lethally and without being detected is sort of difficult, at least it takes a little time to see how to do each part and to understand enemy movement patterns. I mainly used the choke out takedown to silently and non lethally take out most of the guards. This guide explains how I did it.
There is a video at the end of this post containing a visual version of this guide.
Just after you escape your prison cell you see a large room with 3 people in it:
The first is standing with the back to you. Wait until the other two are out of the room or turned the other way and take out the enemy in front of you. Be sure to take him around the corner, although any more hiding of his body is unnecessary. Next take out the guard that is standing next to the right wall. Wait until the other guard is out of the room and this one has his back turned and take him down. Again be sure to hide his body.
The last guard in this area is fairly easy to take down, just wait until he has his back turned.
After some exploring you run into a single guy behind a door:
Nobody is watching him so you can just take him out. There is no need to hide his body. Next if you follow the walkway you eventually run into another guard:
Again, nobody is watching this guard so you can take him out easily. There is no need to hide his body. The next area doesn’t have any enemies in it, you just have to find a safe to pick up some explosives. After that you have to exit the area through a courtyard containing two guards:
Take out the guard that is standing still first whilst making sure to keep an eye on the other. You can sneak around the right side of the courtyard to get behind the first guard. After that just find a way to sneak up behind the second guard, he walks around a lot so it should be fairly easy. There is no need to hide the bodies of these guards.
Next up you have to go through the admissions area of the coldridge prison:
There are 3 guards in this area. Take out the guard that is walking around in the corridor first, when he is closest to the door and has his back turned towards you. You have to hide his body as later more guards enter this area, I used the courtyard.
You can easily take out the guard in the room nearest the door by waiting until he faces the console with his back to the door. I advise that you hide his body back in the courtyard. When you look through the keyhole to the next room you can see another guard. Just wait until he has his back towards the door whilst standing right next to it. Go into the room and take him out.
The last area of the Coldridge Prison is the entrance area. There are two guards here:
Wait until the guard up top is out of sight and take out the guard near you. Be sure to hide this guard’s body, I used the container left of the door although there is a dumpster up top too. Take out the second guard from behind. After you have planted the bomb rush back to the container to the left of the door:
Wait for the two guards to leave the room again and walk out of the main doors. When you jump into the lake be sure to stay under water otherwise you will be spotted. Surface as late as possible and quickly sneak into the sewer entrance.