There are so many possibles and ponderables always in cases like these and you can come up with all sorts of theories, and many times the simplest is the answer. Here that would be an accident, given the terrain.
On the other hand, to get into shadier waters, what better time to disappear someone when you know they are going to be all alone that day, with both daughters out of state and husband away too.
Questions. She obviously had life insurance? Who would benefit? When did husband leave, what was the exact purpose for being away, and when was he due back? How long had that absence been scheduled, for a week or two, or just a sudden thing? Had any strange vehicles or people been seen hanging around in days before she went missing? Did anyone have a grudge against either of them? Were one of them having another relationship?