First, the altitude was above sea level, so it is not clear what it was in relation to terrain. But the odds are good that it was in what we call the "ground clutter" for any ground-based radar (half a mile up at 10 miles distance would be an elevation above terrain of about 3 degrees, farther being less), so it would not have appeared on any surveillance system. Can't see it for the nearby trees or buildings.

Second, it was performing hover flight maneuvers (as they declared). I expect this was a training exercise or possibly an equipment check.