Inside the draw underneath the small statue of the horseman. You need to solve the puzzle with the map in order for it to open. To solve the puzzle, make a note of which numbered sections of the map are not lit up. Then visit all the guest rooms on the 1F and 2F and look for a picture with a switch. For each picture that matches a number of an unlit map section press the switch underneath. Once all map sections are activated the draw will unlock.