Where is the Cell Hallway Key?
There's an actionable door right outside Corvo's cell, but the key wasn't on any of the guards and it wasn't vital to the mission. How can I enter this room? Is it an Easter egg or something? What's inside?
As you progress through the prison, you will eventually make it to the room on the other side of the gate. You can open it from that side. There is no key (AFAIK) that you can find, pick or loot from guards to unlock it from that side.
You'll make it there through normal progression so don't worry about trying to open it.