He had a workman with him.

Monsieur de Marquet had had the walls laid entirely bare; that is to say, he had had them stripped of the paper which had decorated them.
Blows with a pick, here and there, satisfied us of the absence of any sort of opening.

The floor and the ceiling were thoroughly sounded.
We found nothing.

There was nothing to be found.

Monsieur de Marquet appeared to be delighted and never ceased repeating: "What a case! What a case! We shall never know, you'll see, how the murderer was able to get out of this room!" Then suddenly, with a radiant face, he called to the officer in charge of the gendarmes.
"Go to the chateau," he said, "and request Monsieur Stangerson and Monsieur Robert Darzac to come to me in the laboratory, also Daddy Jacques; and let your men bring here the two concierges." Five minutes later all were assembled in the laboratory.