Golden Cat Master Key is a written note found in Dishonored.
I know we're running low on beds, but I must have a room for the child. She's more important than you can guess, and recently she almost got away, using the VIP entrance. That's why I decided to keep the master key in my possession. This girl must be cared for, even if she's a brat, and I want her kept in a room to herself.
Another copy can be found in the basement of the Golden Cat, in the locked room beside the steam room, on a hook on the wall.