I suppose it's possible that it happened like this - they maybe had dinner together, or at least saw each other, then he went out. Maybe they had argued. Either he stormed off, or went to the pub, or was meeting some mates, whatever. It was still light, so she decided to mow the lawn. There was an accident and she died. When he returned, there was nothing to hear (I believe ride-ons' engines stop when they roll), and nothing to see (it was now dark, and anyway Frances and the mower were both on the downhill side of the wall.) It was late, and he assumed she had gone to bed. He did whatever, maybe watched some TV. That might take an hour or two. Then he went to bed, and noticed that she wasn't there. If they had argued before, he might have assumed that she was sleeping in another room, or even had left him. So maybe after a while he started looking for her, all through the house, then in the garage where her vehicle still was. Then started looking around the property, and finally found her. And by this time it was after 3am. Fairly believable - BUT why haven't we heard this explanation?