Stephen,
Look at that wall to hear what Holmes say about it.
I think there are 3 active places (hole on top of wall, bottom right and center of wall) on that wall plus the wall outside by the stairs.
Then use the spade.
White wall: Go to the white colored wall. Check the wall and it sounds hollow.
See the crack at bottom right of wall. Look up to see an opening at top of wall. There is a piece of iron behind the opening.