Every room in our Menifee home is coated in a fine oily dust, but there was no fire. What happened?

That presentation is a furnace puff-back: a gas burner misfires and blows soot through the duct system in a single event, distributing it to every room the ducts serve while the furnace area itself can look almost clean. The fix runs in order: the furnace is serviced and the cause corrected, the ducts are cleaned as part of the loss, and the rooms are cleaned with dry soot methods before any wet cleaning, which would smear the residue into paint and fabric. Wiping a test spot with a wet cloth is the demonstration of what not to do at scale.