Faked his death
There have been many cases throughout history of people faking their own deaths. Don’t ask us why some of them do it, each of them has their own reasons. Some people are fed up with their life and want to start fresh, others have debts that they can’t pay and need to disappear.
This grandfather should have figured out a better way to vanish though, he didn’t leave any fake evidence for people to think he was actually dead. Not the most detail-oriented man we guess, but no one really seemed to be looking for him after his death either. We guess this is goodbye then.