Just like
in January, this is one thing I actually think Occam's Razor really puts this to rest:
1) A car that looks like known Model X mules is spotted outside of the Tesla factory, some weeks or months before the expected Model X launch.
2) The car shows the usual missing trim, the camouflaged nosecone,
everything we know from the Model X mules intimately. It is all there.
3) The car even has falcon wing doors, the signature handles, with the protruding prototype glass, in all the right places. It has that huge front window extending above the front passengers, like on Model X.
4) Last but definitely not least, the years between now and expected Model 3 launch - and the word inside says Model 3 is yet nothing but a clay model, let alone anything to drive on the road - suggests it is unlikely to see a Model 3 on the road.
Now, theoretically, of course it could be a Model 3 platform tester of some sort with metal borrowed from existing Tesla's... but wouldn't that be the easiest done with Model S, not Model X which itself is still in development...
By the way, are you OP sure they were loading the cars... could they have been unloading them? Maybe they had been testing out of state or something.