Regarding a specific hidden event

I've been searching for the UFO easter egg/event for a while now since I found it on a list of missable events but can't seem to get it to trigger. The only things I've found regarding it in-game are two NPCs who talk about it in Rampart, one mentioning increased sightings near the triangular island and another just talking about ancient astronaut theories. How do I get this to trigger? Thanks in advance