never known one go of spontainously, if it was a known fault on that vehicle there would have been a mega recall on it being as its such a safety issue.
There are certain vehicles with side air bags that seem to go off just by side swiping a kerb..(BMW & Clio being 2) Front airbags require an impact at a certain minimum speed and certain angle before they deploy, if you cant see any visible damage it may that they have driven over one of those car park non return type thingys (rising kerb / traffic control plate / rising bollard ) so any damage will be underneath , this does set them off ,seen cars written of due to this
...perhaps they are to embarrassed to own up