Actually, the policemen don't leave the house after investigating her husband Patrick's murder. In the final scene, Mary has invited them to dinner and they accept. Gathered at her table, they eat the leg of lamb she has cooked after she had used it in its previously frozen state as a weapon to kill Patrick. The policemen are literally eating the evidence of the crime. In the next room, Mary hears them discussing how they will certainly find the murder weapon, and she laughs quietly. Obviously, she knows they won't, and she has gotten away with murder. The policemen think the murder weapon is "right under our noses, and they are more correct than they realize.