military radars have not seen anything
Military radars could have seen it but it takes a human being to actually crunch the data and call a target a 'threat' and act upon it. It isn't military business to sort out a stranded traffic. I bet I could fly in a jet over most of Europe, turn transponder off and none would try to pursue me, shoot me down, unless I ventured over very sensitive areas, perhaps I would be met by stern-faced aviation authorities after landing...