When I was flying light twins around East Africa, we would have dead bodies often. Sometimes they were in coffins, sometimes in body bags. One of my colleagues was flying a body down to Mombasa from Nairobi. The front right seat of the Cessna 402 had been removed and the body bag was lying beside the pilot. In the descent the body sat up, let out a long sigh and slowly lay down again. It was only expanding gasses being released but it really freaked him out.