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!"