I just finished the book, (given recently to me by a friend) after having written a novel myself about my family in WW2. The little box/model house ended up with Jutta, Werner's sister, many years later. Soldiers were missing and many years later, charities, volunteers and the military were still trying to locate remains and return property. The son detects it is a box, but doesn't open it. Jutta takes the model back to Marie-Laure, with the key in it. The inference is I think that Werner wnet back to the kannel and retrieved the box, but let the stone slip into the ocean, as Marie-Laure had wanted.
The little box/model house ended up with Jutta, Werner's sister, many years later. Soldiers were missing and many years later, charities, volunteers and the military were still trying to locate remains and return property. The son detects it is a box, but doesn't open it. Jutta takes the model back to Marie-Laure, with the key in it. The inference is I think that Werner wnet back to the kannel and retrieved the box, but let the stone slip into the ocean, as Marie-Laure had wanted.