writes: Tesla is already on the record saying an all-wheel-drive version of the Model S electric car is coming, and soon. That car will share its drive system with the upcoming Model X crossover, based on the Model S platform. But now prototypes have been spotted with some mysterious testing equipment. Notably, two prototypes appear to ride slightly higher than a standard Model S. Tesla's said the all-wheel-drive system in the Model S will be "an efficiency-neutral option," which means it won't reduce the car's rated electric range. Or is this a Model X "mule," given its heavy roof rack? Or some kind of autonomous Model S prototype? Whatever the answer, Tesla's up to something...