On a summer afternoon seven years ago, I was on my way to visit a friend in Bavaria when I got a tip-off about an explosive story in the next edition of Der Spiegel magazine. It was about how Marie Sophie Hingst, a German historian and Intel employee in Ireland, had claimed for years in an online blog that she was Jewish. Her grandmother had survived Auschwitz, she wrote, though many other relatives had not been so lucky. None of this was true and Spiegel journalist Martin Doerry, whose own grandmother was murdered in Auschwitz, exposed Hingst as a fraud.“Anyone...