My wife's a vet and is actually considering starting up a service of just doing this. She is a dog lover herself and was talking about the fact that dogs can smell disease such as cancer and can also smell death very clearly. Most vet clinics, at least here, put the animals down in a room especially for this. That room has a hatch in the floor where the body is put afterwards, sealed in a bag. I can only imagine what that room must smell like to a dog, that knows it is sick. Compare that to being at home surrounded by familiar people and smells.