Thank you, Pers, but we aren't talking about the same room. The one that opens off Nicholas's study is H on the plans on this forum. The room I asked about is between I, Nicholas's bathroom, and J, the white dining room. It is directly behind B, the Empire drawing room.
It may also be a passing room, but the plan shows something in the middle of the room. It doesn't look like a staircase. I think it may be some sort of built-in furniture