During your playthrough of The Surge 2, you will run into a few side quests you can do. Some of these are easy to complete, and others have hidden objectives.
You can find Rex again at the top of the Power plant. Start on reactor level 2 and on the catwalk there is a railing missing and across a small gap a door that can be forced open.
Go through the door and you are in a tunnel of sorts with a container truck and a Spark Fanatic, to your right is a force lift, take the lift up to the platform. To your left when you land is a mag lift go up that, you will be placed on a catwalk on the outside of the power plant, follow it around to the left. There will be a door to your left, don’t go in there yet.
Go straight and to the left where there is a item Chest and a SPARK aspirant. Now go back to that door and take the stairs down to where the 3 Enemies are, kill them. There is a door to the left of the Hologram and opposite the door are stairs going down 1 more level to a item chest. Go through the door and turn right then go up the stairs.
You will find Rex locked in a cage at the top of the stairs and if you talk with him you can choose to start the side quest Liberation which has you find a key from the prison guard to free Rex.