" "Here you all are, and good friends of dear Patrick's too, and helping to catch the man who killed him." " (Dahl 119)
" "Please eat it. Personally I couldn't touch a thing, certainly not when it's been in the house when he was here. But it's all right for you. It'd be a favor to me if you eat it up" " (Dahl 119)
" "Personally, I think it's right here on the premises" " (Dahl 119)
" "Probably right under our very noses. What do you think, Jack?" " (Dahl 119)
Mary tells the policemen would be doing her a favor by eating the meat because she couldn't possibly eat it by herself since it was in the house when her husband's dead body was. But when Mary said that they would do her a favor, the policemen would be doing her a favor by getting rid of the weapon that she used to kill her husband.
We expect the detectives to find out who the killer was, since they were so experienced, but Mary Maloney outsmarted them. It's hard to expect that a women would outsmart trained professionals in the detective field.
While eating the lamb leg that Mary Maloney cooked for them, the policemen said that the weapon could be under their noses, but they don't know that the weapon literally is under their noses while the audience does.