statistically, the risk is probably buried beneath a myriad of other things that are more likely to harm.
I think the OP may be asking for the "how could it happen?" he's mentioned it a couple of times.
1. gas vapor from fueling process, or spilled fuel on the ground becomes the flame source. VAPOR is the far more dangerous, as liquid unleaded is actually stubborn to light.
2. sources of ignition kick it off. Easy ones are cigarettes, static discharge.
I think newer vehicles provide less opportunity for ignition sources than old. When is the last time you heard backfiring from burnt/leaky valves, or through-the carb (true) backfiring that we used to have years ago? My old chevy would occasionally burp flames through the air cleaner.
There are still risks. my mower usually has a pronounced backfire 5-10 seconds after shutting it off, after the engine stops spinning. it's new-ish.
The newer vehicles tend to have no distributors, and relays tend to be sealed in a box.
List of possible sources obviously include cats, arcing relays, switches, motor brushes. Cats are hot. Any exh leaks, especially at the manifold, would be a big risk. Plenty of late model vehicles are still known for manifold cracks.
It would be far more likely that a vehicle with a fault, such as arcing plug wires, loose connections carrying real power, would be a potential spark source, than a newer, healthy ride.
Nearby OPE, especially if the spark arrester has been removed.
Gas station mechanicals or wiring.
Static discharge from nearby electrics... motors, HVAC contactors, Car wash contactors, static off of metal halide lamps, etc...
and the vapors would have to be concentrated enough.... minimal wind, humid conditions making it slower to stratify.... which also makes static discharge harder to happen.
it would take the right conditions...
Still, I shut mine off. If the car isn't moving, I power it down. Outside of the safety... it's my money idling away.
