At the end, it's revealed that Frank Bennett's body was cooked up in the barbecue (that's how they chose to dispose of the body.) But I've read several true-crime stories in which humans were indeed cooked, and I've also read accounts of the death camps in Nazi Germany were humans were incinerated. All of them have in common that they describe the smell as awful and unmistakable. The neighbors would smell it, and even if nobody knew just what they were smelling, there'd be major suspicion falling on the diner.