Skunkworks & Northrop Grumman sites are all within a 100 miles of the sightings. That's too much of a coincidence for me. "Highly advanced" projects coming out of those locations, usually run decades before anyone external even hears about them.
edit: but to answer your question, a meteor.
- The operator is lying or otherwise mistaken
- A radar jamming system under test
- Noise and/or weather effects
- Multiple objects
- Cyber warfare
This isn't to say that I don't think extra-terrestrial intelligence isn't probably out there. I just think it's extremely improbable that they made it to earth.
I also think it's possible that the measurement device reported an object, and a human saw it, but the device wasn't measuring something that was there. Either by malfunction or sabotage/stealth technology.