Hi Peter, I wonder which key you're talking about? (Maybe I shouldn't have put so many keys in the game...)
There's a key to get out of your cell.
There's a key to open the evidence locker.
There's a key given to you by a prisoner in exchange for something, that one isn't labelled but (spoilers) it opens a room a few rooms north of that prisoner.
And there's another key that opens the high tower cell.