I used to fly Super Pumas in the North Sea. Once after landing on a platform, after a 15 min sector from another platform, it was clear something was amiss in the cabin - I was waiting by the door for pax to disembark. Entering it was easy to spot that one of the pax in the penultimate row, aisle seat, was clearly dead. The neighbours stated he’d boarded normally on the previous stop and almost immediately gone “to sleep”. After buckling up. Nobody realised what had happened until the rear seat guys tried to get past after landing.
It’s shocking how quickly it can happen.