Answer: Detective Noonan’s comment made Mary take action and eliminate any proof that she was the culprit.
Explanation: As detective Noonan was getting to a conclusion that the culprit's item might have been a big spanner, he implied that finding it would lead them to the author of the case, that is the reason why Mary offered them some drinks and insisted on them to eat the lamb she (intentionally) had left in the oven. Mary acted carefully in order for the policeman not to notice they were actually eating what they were looking for.
