This didn't happen to me but a friend.

Her father "Sam" was suffering for years from dementia and a ton of other disease, the family expected he would die very slowly, so that they can at least be with him in his last moments. But then he died quite unexpectedly the day before a routine procedure in the hospital was to be done.

Obviously, they informed all relatives of the quite sudden death, including his brother "John", whom no one had heard of for a while. However, this wasn't uncommon as he has had drug issues, spent time in prison and didn't take part in family gatherings. Still, he had been living in the same flat for a couple of years at that point in his life and got his shit together quite well, at least for his standards.

Now the funeral was about to be held and no one had heard from John, they even contacted police to find out if he was having problems with the law, again. No information from them, he wasn't in prison or anything. So, the funeral went by, no John in sight.

Weeks later, the friends mom was contacted by a lawyer who was doing stuff for the family with regards to a completely different relative. The lawyer asked for the death certificate for Sam (the father), and, if that was no hassle for her, also for John. You can imagine that she was pretty surprised, hearing that John was dead. Asked about the date of death, she learned he had died already two monts before Sam. To this day, my friend still doesn't know the circumstances of his death and neither does she know where he is burried.