Tommy Rojas tweeted this pic: tommy rojas on Twitter: For me it looks like a dual-motor Model S with wheel sensors hauling a trailer loaded with test load. TM doing some testing of a trailer hitch? The D versions of Model S do have the same motor combination as Model X, that will be available with trailer hitch. It seems like they are preparing to release it as an option to at least the four-wheel drive versions of Model S too.
A poster on here has previously mentioned having an Eco-Hitch installed on their MS. Don't know what the load rating would be though. Perhaps that is what you saw? Torklift Central | 2012-2015 Tesla Model S EcoHitch® - Tesla - Find Your EcoHitch® - Trailer Hitch and Towing
That's the new extended range option they're hauling around... a second 135kW battery in the trailer. Phenomenal range, and even more luggage capacity in the top half of the U-Haul. Heard they integrated a motor into the trailer, which reduces the hitch drag load quite a bit... and of course, uses lightly configured regenerative braking (but only to charge the trailer pack and not the main pack on the car, pity).