Drums in the Night
One night, after the NPCs looking for the grain go missing, all the remaining colonists are woken up by loud drumming. It sounds like it’s coming from the mainland but it’s impossible to be sure. It’s also sounds like there are other drums answering from channel to the east of the island.
This goes on for several hours (3) and the suddenly stops. It’s deadly quiet with no animal sounds for about 30 minutes when things return to normal.