Wisconsin is far from the most densely populated area in the country, it's entirely possible that a military airbase in the region was playing with a new toy. If I were a test pilot, I would enjoy flying where I could just barely be seen and mystifying civilians.
It's not mysterious, but the coolest airborne object I've seen was (on the evening Gulf War II started,) I was out walking and a F-117 Stealth Fighter was passing overhead low enough that I could see the tail markings as it banked to turn. I estimated at the time that it was only about 800 feet above the ground (the ground is ~60 above sea level.)
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