This happened not far from my house a couple weeks ago:
[video=vimeo;59564508]https://vimeo.com/59564508[/video]
A 1991 M5 - the last of the hand-built M5s. Couple hundred grand on the odometer.
The owner had driven just about a mile from his house when he saw smoke. He thinks it was leaves/pine needles that fell through the exterior air vents in front of the windshield that ended up resting next to resistor-based potentiometers that get hot when used to slow down fans. That doesn't seem right for only a mile of driving. The fire was surprisingly slow to get going (water & fire extinguisher didn't work by the time we got them to him), but I think there had to be some kind of fuel/oil leak to have it spread like it did.
Oh, and no boom. Apparently, in most cases something rubber burns away and the fuel drains out of the tank onto the ground. The big danger is an almost empty tank that has a lot of fumes, which are more easily ignitable.