"_____'s Spirit is Fading Away" Message. What is this?

I got this message for the first time today also, I am looking for one sim who is part of my legacy family, but I have no idea how to locate his tombstone, since it is not in my family graveyard lot, so I need to figure out where he died, I have found some tombstones in some of my family sim lots but not his yet, I wonder if his tombstone is at his workplace, hmm. I wish they told you where to go to find the tombstones to help them XD