I remember that the LS bodies with the 3.5 had a recall for leaking fuel rails. More than a few of them had underhood fires.That could do it. My first wife drove our car to the bank one day. As she was at the teller window, bank staff started looking out the window - one grabbed a fire extinguisher.
Yes, our car had a small fuel leak that happened to hit the exhaust or something and there was a minor fire. Luckily they put it out. I had my own shop then and submitted an estimate for repair and insurance paid me to fix it. Didn't take a lot - mostly fuel lines and plastic parts......
All it takes is a few drops to vaporize under the hood.