If it is a premade that came with the game/exp./gp/stp then the internet should have their location. There is a wiki page dedicated to the Sims 4. I don't have the URL off hand and can't access my bookmarks atm but if you do a search for his name (just remember to include sims 4 in the search string) it should pop up. There you will find his or her location.
If you have MCCC thing which I think you do based on this thread, you might be able to access his info. I have no idea because I don't use that particular mod myself but considering what it can do it might just keep profile records of deseased Sims. I do know that you can access hidden household that hold special NPCs like Death, the adoption pools for both Sims and pets and ghosts. So maybe that will help. I do hope you find your urn.
