During the final mission of Chapter 1, you will finally find your husband. Just before finding him you will have seen a strange room with a large table and what looks like a door with 3 bars in front of it. This is where Shelby Cross is hiding and there are 3 levers you need to interact with to open this secret room.
The first lever is behind the cage with your husband in it, you can shoot it with a rifle from across the room.
The second lever is down a path beside the stairs in the main room with the locked door, (there should be rubble you walk through) The lever is in the left corner of the space.
The third lever is in the big room with all the enemies. High on a ledge in the corner. You can shoot this lever as well without getting close.
See the video for exact locations
Once open you can find Shelby and kill him. I suggest sneaking up on him and knocking him out first as he has a LOT of health. Once knocked out you can put him on the ground and shoot/stab till he's dead.
Full run through in the video (including ALL stealth to get the Great Escape trophy as well.