Mark One: The Adventure of the Black Door
Nudge. This is the night after the one you spent at the hearth. Where did Phoebe take you, at the end of it?
Harder
In 1665 you hid the back-door key where Pike's search would never reach: in the stair itself, in the one part of it that is hollow and has a cap that lifts.
The mark, in words
The hall has five places a key could be, as five buttons: The kitchen hearth, The stair treads, The newel post, The panelling and The chimney. Tap one and then Look there.
The answer
The answer is newel.
In Part 1 you hid the back-door key in the hollow newel post at the foot of the great stair. The newel is original, the cap still lifts, and the key is still there. The hearthstone, the treads and the panelling were all replaced in 1912, and Pike searched the chimneys.