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.

Achille gave a cry of amazement. But Lupin objected:

"Well? And what then? We are no better off than before. Here is an empty
oblong, eight or nine inches wide by sixteen inches high. You're not
going to pretend that a woman can slip through an opening which would
not admit the thinnest child of ten years old!"

"No, but she can have put her arm through and drawn the bolts."

"The bottom bolt, yes," said Lupin. "But the top bolt, no: the distance
is far too great. Try for yourself and see."

Achille tried and had to give up the attempt.

Lupin did not r