I defeated Penguin and Black Mask and rescued the warden. The warden needs me to find the secret entrance to his office back in the industrial portion of the prison. The thing is I have no idea where the secret entrance is. I think it has something to do with the water pipe in the sewers that I couldn't get past before right before the Solomon Grundy boss fight. I have no idea how to get back there. What do I do? Did I have the wrong idea?