I don't know if this applies to your situation or not. If the child was adopted or had a parent that somehow died and they were younger than a teen, then they end up "back in the system" so to speak. I've had this happen and it seems for me, at least, it has been this issue. Teens can live alone, but anyone younger can't. I have no idea if this helps explain what you're experiencing. Figured I'd give you what I found to be my experience with missing children from the relationships panel.
Aw man I don't know if they were a townie or a random not-living-in-the-neighborhood sim but that may be why she's gone 😕
Community ManagerHey there @673047,
This Sim may have been "culled" or removed from your game. Sim culling is a performance-management mechanism- the game will automatically delete unplayed Sims or households to reduce the save game size. There is a set number of Sims you can have in your town at a time, and once that number is reached, random unplayed Sims will start to be removed from your game. Unfortunately once a Sim has been culled from your game, there is no way to get them back.
