After Mary kills her husband and ensures that she has an alibi, she gets officials involved, calling the very detectives that her husband worked with to the scene of the crime. As the detectives and other officials continue their search, the proceedings become more and more centered on the question of the murder weapon, which was "almost certainly a large piece of metal." Mary overhears them discussing it, and at one point, detective Jack Noonan brings it up to her directly. He even tells her "Get the weapon, and you've got the man."

It is at this point that the reader knows the question of the weapon, the leg of lamb, cannot be ignored. The investigation around her is all but forcing Mary to act if she does not want to be discovered. The reader knows she must destroy the evidence.

As for the weapon being consumed specifically, the reader does not necessarily know for certain that this will happen until Mary actually offers the cooked leg of lamb to the officers. It is, however, hinted at when she puts the lamb in the oven earlier on in the story.