Yup - we also used to see the Designworks guys loading car-sized "somethings" into trucks and trailers. These were often very light - a few guys could pick them up and walk then into a trailer. I always assumed they were some sort of mock-up. I've also seen a few odd looking vehicles on the coast freeways - these usually have mfr plates and all sorts of mysterious electronic boxes attached to the dash.
They clued in finally and enlisted the Hollywood factor. You can do a complete overshell that weighs nothing.
Mike Nicols used to get a kick out of the fact that when they were filming Catch 22, at the end of the day a couple of crew would come over and hoist the Sherman Tanks on their backs and carry them off to storage.