You mean--" "I mean the key of the room upstairs.

You said you found the key in the passage outside.

You must have locked the door after you and taken it away with you." "I did," replied the young man, in some hesitation.
"For what reason ?" Charles realized that he was on very thin ice.

In his intense preoccupation with Thalassa's story he had forgotten that his own impulsive actions on that night must be construed as proof of his own guilt or bear too literal interpretation of having been done to shield Sisily.

He saw that he was in a position of extraordinary difficulty.
"I was hardly conscious of what I was doing, at the time," he said.
"You took the key away with you ?" Charles nodded with the feeling that the ice was cracking beneath him.
"And how did it get back into the room afterwards ?" Charles paused to consider his reply, but the detective supplied it.
"The inference is fairly obvious," he said.