VW owns property several magnitudes larger in area than Tesla, including the gigantic Ehra-Lessien facility in Germany where the Veyron can stretch its legs. And yet we still see pre-pro VWs on the streets all the time.
And we see spy photos showing other makes all the time, too, even from remote locales like the ones you mention.
I think the explanation above--that there aren't yet any pre-production cars--is much more likely. Car makers invariably build cars for testing and evaluation on production lines before production begins (though as someone else mentioned, there are plenty of places that will build one-off prototypes, too).
The driveline is similar enough to the Model S that they can probably do most of the basic development with the cobbled together Model S mules we've already seen. There may be other disguised drivetrain prototypes out there--wasn't the Model S developed under a Dodge Magnum, back when it was still "Whitestar?"
As for drives? Other companies frequently do press drives of pre-production cars. Tesla may do the same, though they may also figure they don't need to bother because the buzz is already tremendous.
