There was mystery here, at least, a mystery beyond his power of discernment.

However, this recognition rather hardened him to his task, than otherwise.

He had been forced into the strange environment, and now meant to penetrate its every secret.
This time he locked the outer door carefully, and lay down on the bed, wondering if there would be any further developments.

As he attempted to think, he was listening eagerly for the slightest sound of movement in the hall.

There were none; the transom stood partially open, but no noise reached his ears from the outside; clearly enough the night prowler, assured that he was still awake, had decided to make no further effort.
Doubtless she believed her escape had been unseen, or, at least, that she had remained unrecognized in the gloom, and would now resort to some entirely different method for achieving her end, whatever it could be.