There could be only one way.
Swiftly he dashed to his trunk, knelt and examined the lock, unscrewed it, and took out the documents more precious to him than the treasures of a hundred Captain Kidds.

Instantly, he returned to the window.
Nothing was missing.

But here was something he had never noticed before.

On the face of the slip of parchment--a diagram, dim and faded--was an oily thumb-mark.

The oil from the lock; nothing more; doubtless he himself had touched it.