Golden Cat Master Key is a written note found in Dishonored.
Transcript[]
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.
Location[]
It can be found on the desk in Madame Prudence's office in the Golden Cat during the mission House of Pleasure.
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.