Dead Sim Grim Reaper and wife don't know
That does sound like some type of weird bug. But if you have Neighborhood Stories active, sims you know can and, at least a few, will get killed off. So, you can take a look at and adjust your NS settings in Manage Households to protect sims you want to stay in the game.
Since you already had it happen but had him call you to meet up somewhere means he is still in the game and can be saved by asking him to join an active household. Typically, when a sim dies, the ghost will be culled in 90 game days. But you can resurrect him by one of the resurrection options in the game if you can do it within the 90 day window. Ambrosia, Book of Life, wishing well and the Dedeathify spell are the ways I know but there might be one more I've forgotten. The latter is the easiest as all you need is a spellcaster that has learned the spell. After you have the sim in a played household again, he will be safe from being culled and you can take all the time you need to resurrect him. So, if you can find him out the world somewhere, you can save him from culling. You have to use either the ask to join household or the Invite to join the household option to pull it off in person since ghosts usually are hidden and you can't do a direct move via MH.
Hope this helps.
