I had no other thought.
The slowness with which he groped his way forward indicated unfamiliarity with the apartment, although his direct advance proclaimed some special purpose.

Clearly he had no fear of attack, believing no one more formidable than a girl was there to oppose him.
The darkness, perhaps, and silence, convinced the fellow that she had already retired.

He would have his grip on her, before she could even dream of his presence.

Then there would be no scream, no alarm.

I could determine almost his exact position as his advancing foot felt cautiously along the deck, seeking to avoid striking any obstacle in the darkness.