Secret Public Events
Secret Public Events are random PvE activities in public Sectors that any Guardian can join. Unlike the regular Public Events, these are not marked on the map and players can't fast travel to them. However, they are also much smaller in scale.
Secret Public Events are announced only by a text message in the lower-left portion the player's screen, and not every time. It's up to the player to locate them, so coming across such an event is often a matter of luck.