White Spot by Murray Leinster — Read

It was a queer place, they soon learned. The climate was cool, but pleasant. There were no radio waves beneath a readily detectable ionosphere. Yet apparatus over an area three hundred miles by an average sixty -- the white spot -- had responded in seconds; in parts of seconds.

Which meant electric control. Which implied radio. But there were no radio waves, which should have been proof that there was no civilization on this planet capable of doing what certainly had been done. Which was nonsense.