Sim died but no Grim Reaper - Just disappeared?

My sim died (I assume of old age?), but there was no notification or anything.  

I was playing as another sim whilst the sim who died was walking the dog and she suddenly had a widow moodlet and he had disappeared from the game.  He is no longer in her family tree?

There is a grave stone in my family inventory, but shouldn't he still be in the family tree and in her relationships panel?  There are loads of ghosts there of her friends who have died, but not her husband?