How about a serious theory?
The major problem facing us here on Earth is a shortage of clean energy. If we could put something 121,536 km in radius into orbit around the Sun at roughly 7.2 million km distance, the sunlight falling on its surface would be able to generate more than 100 times the energy per square meter than reaches Earth, or about 46,395,708,788,736,000 HP, give or take a few million. Even with only a 5% efficiency, that's still 2,319,785,439,436,800 HP, or 1,730,559,937,819,852,800 watts. You could really light up a few pubs with that.
Now, if we assume an intelligence capable of making a flat or concave disk of that size, it is reasonable to further assume they've got the technology to keep it flat-side to their own star, and to beam the collected energy to pretty much wherever they need it within their own planetary system. We can further guess that most of their heavy industry is in space, and not polluting the surface of their home planet, since they have obviously solved the "beanstalk" problem
A dense, circular metal plate could easily appear to be a less-dense sphere if you're expecting to see a sphere. This *might* be evidence of a distant civilization.
Or it might be the plate of birth of the Flying Spaghetti Monster.