I know this sounds crazy but...
What if one of these test drivers was held up at gun point, forced to give up the car or else they're dead? Then the robber takes off with the car to a desolate garage/area of some sort.
Next, they remove all the panels and viola, the new design is revealed. Then the pictures are taken.
After the pictures are taken, the car is burned so that the evidence is gone.
I know its far fetched and crazy but.. I could see it happening.