Depends on which key you are talking about.
If you are talking about the first key you obtain in the room with the fireplace, it is used in the same room. The white desk.
If you are talking about the second key that is obtained from the office, it is used in the main hallway.
An important note, when unlocking a locked door/shelf, do not press the unlock button again is it can cause the door/shelf to lock itself again and not be opened after.
I hope this reply is useful to you and I hope you enjoy my game.
It is my first so any feedback would be much appreciated