Old room.

The Master Key of the estate was kept under the care of Mr. Jones and when earlier the maid came running asking for it, he simply had what she needed it for. And since Arwen hasn't explained much to her, she simply has said that the lady has asked it to open one of the mansion's doors. 

And since it was Arwen asking, Mr. Jones had handed the master key to the maid. But just when she was leaving, he had told her to stay with the lady to see if she needed any more help. 

He hadn't given much thought because, since the very first day, Aiden had instructed them to give Arwen full access all around the estate. So hiding anything anywhere never crossed his thoughts until he saw the maid coming back.

"Did you not stay with the lady to help?" he had asked the maid. 

The maid had politely bowed her head and said, "I asked Madam, but she asked me to leave as she didn't plan to do much. She just mentioned that she is there to take a look."