I found a key next to a dead body called the alley stash key. I have looked around the area for a door that it unlocks. Where are you meant to use the key? I don’t want to move onto the next area in case I can’t go back.
You will have to backtrack a bit. You will have discovered the alley stash at the very start of the game. It’s a metal gate with contraptions inside.
Go back the direction you came. You will have to deal with bobbies if you didn’t kill them already. You will get to a courtyard with a zapper. Head up the steps to the left and the gate is immediately left. Check out the video below for a visual guide on where to find the alley stash. You will get a trophy/achievement when you unlock the door.