Forum:My Sim has returned to the Netherworld, but there is no grave?

I resurrected my Sim as a ghost with the "Oh My Ghost(?)" opportunity. He grew old again and returned to the Netherworld, but there was no urn/gravestone. I have also been to the Mausoleum multiple times, and his grave isn't there. Initially I thought that it was a normal thing for the grave to disappear after he returned to the Netherworld, but I've seen people saying the gravestone should appear where he last was.

But now, the game keeps reminding me that he "is getting older and won't live forever" even when he's nowhere in the game (I think) and that I should visit him. Does anyone have any idea how to bring him back, or where he might be? I've been googling everywhere for this, but I haven't found a solution...