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After retrieving the Altered Biocell, speak with Flossy in Cell Block A, mention that you've found it, agree to deliver it to Red Shoes, and your next main objective will require you to find Mejia in Cell Block B. For the purposes of this achievement, you must reach Mejia's location without raising any alarms. This may sound difficult, but since you will likely be stacking "Code of Conduct" on this playthrough, sneaking past hostiles and avoiding confrontation at all costs, remaining undetected shouldn't be too much of a problem. Remember: prison guards and cameras can become suspicious and enter the "search" state. The achievement will not be void until an actual alarm is triggered and the enemy goes from "Search" to "Hostile." In the event you do trigger an alarm, make sure you are reloading your latest save files.

With that said, leave Flossy's cell and follow the objective waypoint on your HUD. It will lead you to Cell Block A's main gate, which can be opened with the Access Card that Flossy gave you (assuming you agreed to his request of delivering the Altered Biocell). Open the gate, enter the threshold, and then open the secondary gate. Take cover on either side of the wall and take note of the camera directly ahead on the Security Office's wall.

Make your way to the left of the Security Office and take the short flight of steps down to a metal door that leads to the service tunnels. Rather than following the tunnel all the way down, look to your right and you'll see a pile of boxes. Move the smaller of the boxes to the ground and it will reveal a ventilation shaft. Open the grate, crawl through, remove the fake panel at the end of shaft, and you will find yourself within a cell on the lowest level of Cell Block B.

You're close to your objective! All you need to do from here is cross the ground level of Cell Block B and reach the showers on the other side. The only problem is that there are two guards between you and the showers. Leave the cell you're currently in and move to the far right side of the room, taking cover behind the pillars as you see fit. Wait a bit while the guards are conversing. Eventually, they will disperse and you can use the pillars and semi-walls as cover as you move towards the gate that leads to the showers. Once you're at the end of the room, wait until the guard here has back turned to you and the camera is looking away. When the coast is clear, open the marked gate with the Access Card, sneak inside, and move down the steps to finally reach the showers.

The achievement will unlock upon entering the showers, provided you did not raises any alarms on your way there.