It's true, radar would have picked it up. English airspace is incredibly crowded, considering the size of the island - unless we are using the "Muggles can't see it because it's magic" excuse which I'm not falling for. I can beleive it for the Knight Bus because it's a charmed object able to leap from place to place and cause objects to leap out of its way.

However, if people could spot a little bitty blue Anglia in a blue sky which flickered in and out of vision, there's no WAY they'd miss this. *Infuriated*