He risked being captured,--and he knew it.

And he was very nearly captured.
He had not had time to regain his post, as he had certainly hoped to do.

He had then a very strong reason for returning to his room.

As for myself, when I sent Daddy Jacques to the end of the 'right gallery,' I naturally thought that Larsan was still at his post.

Daddy Jacques, in going to his post, had not looked, when he passed, to see whether Larsan was at his post or not.
"What, then, was the urgent reason which had compelled Larsan to go to the room a second time?
I guessed it to be some evidence of his presence there.