User blog comment:TheReturnOfTheKing/The Zi'vani and the Chaos Crisis/@comment-1743498-20130929201541/@comment-1743498-20130929202207

A casual observer might suddenly notice the stars blinking out if the Anathema happen to be close - it should be noticeable for any star that happens to be within 15 lightyears. A civilization with more advanced telescopes may detect unnatural anomalies, such as decreasing mass of distant objects.

They may not know that these anomalies are the work of the Anathema, but it would nonetheless be unsettling.