Random Active Sim Death, No Grim

I was playing a household of 5 Sims and 1 pet and while I was not actively focused on the Sim in question, I am confident there was no reason for her to die. I received no warning message (i.e. "Sim is dying of X."), Grim did not appear, there was no collapse or event (fire, meteor, etc) - I just got the "Sim has died" popup asking me if I wanted her to be a playable ghost or freeroaming ghost. She had high needs, was a young adult, and was just mid-conversation. She was in a playful mood but I don't think she was hysterical, and again, I didn't get a "dying of laughter" notification. Just the ghost popup.

There is an elder Sim in the household who is about to die of old age; not sure if she somehow got his death or if this was a coincidence.

I already exited without saving to get her back so I don't have a save file to share with this incident.