In the game "HALF-LIFE 2". There exists an ongoing mystery, that has never been solved. The INTERLOPER Mystery.

INTERLOPER

The INTERLOPER is a mystery surrounding the Source Engine dating back from a discussion thread from 2005, where attention to it first began to skyrocket. However, in the intervening years, leads would dry up and the mystery itself became lost. This was until a group of video game investigators, drawn to the mystery by lead investigator Anomidae, dove deeper into this dark and mysterious void of information through a series of videos re-exploring this mystery that had remained unsolved for nearly two decades. But as the investigation continued, it soon became clear that this was something much grander than the forgotten urban legend that it was on the surface.