The projection of the terrace had prevented my seeing it.

Thanks to that ladder, it was quite easy to get into the 'off-turning' gallery of the first floor, and I had no doubt of it having been the road taken by the unknown.
"We ran to the ladder, but at the moment of reaching it, Daddy Jacques drew my attention to the half-open door of the little semi-circular room, situated under the terrace, at the extremity of the right wing of the chateau, having the terrace for its roof.

Daddy Jacques pushed the door open a little further and looked in.
"'He's not there!" he whispered.
"Who is not there ?" "The forest--keeper." With his lips once more to my ear, he added: "'Do you know that he has slept in the upper room of the donjon ever since it was restored ?' And with the same gesture he pointed to the half-open door, the ladder, the terrace, and the windows in the 'off-turning' gallery which, a little while before, I had re-closed.
"What were my thoughts then?
I had no time to think.

I felt more than I thought.
"Evidently, I felt, if the forest-keeper is up there in the chamber (I say, if, because at this moment, apart from the presence of the ladder and his vacant room, there are no evidences which permit me even to suspect him)--if he is there, he has been obliged to pass by the ladder, and the rooms which lie behind his, in his new lodging, are occupied by the family of the steward and by the cook, and by the kitchens, which bar the way by the vestibule to the interior of the chateau.

And if he had been there during the evening on any pretext, it would have been easy for him to go into the gallery and see that the window could be simply pushed open from the outside.