My sister called me at midnight and whispered, “Turn off every light. Go to the attic. Don’t tell your husband.” I thought she was losing her mind — until I looked through the floorboards….

final idea is what stays with you:

She didn’t miss the signs because she was careless.
She missed them because there were no obvious signs to see.

That’s what gives the story its edge.

Not that danger was present—but that it was convincing.