Since carjacking is so prevalent in SA, just about every car has an anti hijack alarm/immobilizer.
If you don't push a switch within a few seconds of starting, then 30 seconds later the alarms goes off, followed but the complete cutting of electrical power a few seconds later, very similar to what Jim is speaking of. It allows the perp to get away with the car (away from you) before anything happens, that way he doesn't just turn and shoot you on the spot.
Very very effective, although when I worked at a tyre shop and the customer forgot to tell us where the switch was, it was a royal PITA! Had to disconnect the battery to stop the danm alarm!
Tim