He stood motionless, a little pale, with drawn features. There was no doubt about it: the letter was from Gilbert. It was the form of address which, by Lupin’s orders, Gilbert had used for years in corresponding with him. Gilbert had at last—after long waiting and by dint of endless artifices—found a means of getting a letter posted from his prison and had hastily written to him. And now the letter was intercepted! What did it say? What instructions had the unhappy prisoner given? What help was he praying for? What stratagem did he suggest?
Lupin looked round the room, which, contrary to the drawing-room, contained important papers. But none of the locks had been forced; and he was compelled to admit that the woman had no other object than to get hold of Gilbert’s letter.
Constraining himself to keep his temper, he asked:
“Did the letter come while the woman was here?”
“At the same time. The porter rang at the same moment.”
“Could she see the envelope?”
“Yes.”
The conclusion was evident. It remained to discover how the visitor had been able to effect her theft. By slipping from one window to the other, outside the flat? Impossible: Lupin found the window of his room shut. By opening the communicating door? Impossible: Lupin found it locked and barred with its two inner bolts.
Nevertheless, a person cannot pass through a wall by a mere operation of will. To go in or out of a room requires a passage; and, as the act was accomplished in the space of a few minutes, it was necessary, in the circumstances, that the passage should be previously in existence, that it should already have been contrived in the wall and, of course, known to the woman. This hypothesis simplified the search by concentrating it upon the door; for the wall was quite bare, without a cupboard, chimney-piece or hangings of any kind, and unable to conceal the least outlet.
Lupin went back to the drawing-room and prepared to make a study of the door. But he at once gave a start. He perceived, at the first glance, that the left lower panel of the six small panels contained within the cross-bars of the door no longer occupied its normal position and that the light did not fall straight upon it. On leaning forward, he saw two little tin tacks sticking out on either side and holding the panel in place, similar to a wooden board behind a picture-frame. He had only to shift these. The panel at once came out.