Tam has the details.
Me: Mass hallucination is my guess. All those folks wearing rose colored glasses (it is LiberlLand, after all) must have caused some kind of refraction in the reflection of the sun from those self same glasses.
I like Tam's explanations better, but who knows???
ETA: Via Insty we get this, a much more rational explanation. Sadly, the military seems clueless about this idea. One would think that they could have pulled radar records and matched this flight fairly quickly.
Then again, perhaps they want to keep everyone guessing.