If nobody appears, pop over to the nearest houses. Chances are, the car belongs to someone visiting friend three doors down. Knocking on the door with a smile—even a forced one—works wonders. Say, “Hi, so sorry to bother you, but is that silver Fiat yours? Seems someone’s just nipped into my drive.”
Most people will be mortified. They’ll apologise like you’ve caught them stealing biscuits. You suddenly hold the moral high ground, and they’ll move it faster than if you’d yelled through the letterbox. It’s a win for dignity, and you get a free dose of smug satisfaction.
