I just had the most disturbing thing ever happen. My SO's daughter came out of the room where he's…

I just had the most disturbing thing ever happen. My SO's daughter came out of the room where he's talking to her mom and started crying, say her mommy had killed her daddy. Full on bawling. I've no clue where she even got the idea of someone killing someone, much less saying that. Everything is fine, but she's been having issues with anyone leaving all day, even herself. Said she couldn't come home if she went to school this morning. Keeps saying that someone will leave and never come back. Keeps making sure that we say it's "her" daddy, mommy or whoever it is, you have to say they are hers when talking about them. And now this. Needless to say, I'm very concerned on where she's getting these ideas as it seems like more than just her not wanting to be alone or left. I've no clue what to do. Told my SO and her mom and they're just as lost. She's had a basic grasp of death, both her grandparents are dead on her mom's side, but it's just the fact that they can never come back. No clue where she'd get murder or that her mom would kill her dad... What if she says things like that at preschool?