The German town of Gangelt is the study case that the OP was referring to, which thanks to a small population and a well-attended carnival party with many infec

The German town of Gangelt is the study case that the OP was referring to, which thanks to a small population and a well-attended carnival party with many infected attendees and a legion of subsequent linked cases had acquired the reputation that everyone must have been exposed, i.e. a place that would have established something like herd immunity. So to discover that only one in seven inhabitants had detectable antibodies was indeed dismaying: either the infection pattern or the immune response wasn't as expected. Or both.