Tombstone is probably where the sim died
therefore somewhere near one of the rock climbing wall in Mt.Komorebi? since he died by falling?
although I don't think sims should be dying while you are playing other sims
so maybe check neighborhood stories n such settings
could have happened I suppose while you were visiting the neighborhood with another sim but uh still strange
that luckily has never happened to me with anything else than temperature deaths
if its neighborhood stories I think the tombstone/urn is always on sims home lot
that is if the sim has home lot
if neither then honestly idk
you have to find the tombstone/urn before you can do anything about it