[MontagFTB]

The shadow cast by the tarp suggests it's opaque, not transparent (or even translucent, really.) Combined with the horizontal lines across "plane", it all suggests a weather-beaten opaque tarp that's been painted to conceal its contents and confuse any overhead photographers.

[eightails]

And there's also the fact that the other possibility seems so unlikely: why would you store a sensitive experimental aircraft under a transparent tarp? I can't think of any reason to do so.

An opaque painted tarp seems much more likely.

[abliefern]

This is addressed in the article. This could be a standardized military hangar with the back wall put in but no roof tarp installed