The Invid Sensor Nebula was a creation used by the Invid to search the cosmos for traces of Protoculture and the Flower of Life. Once detected, the Nebulas transmitted its findings to the Invid, who then arrived to invade the targeted planet.
During the Second Robotech War, a Sensor Nebula drifted near Earth during the time when the Robotech Masters assaulted the planet in order to claim the Protoculture Matrix. This was a disturbing matter to the Tirolians who knew that it was only a matter of time before it took its data to its Invid creators. They also learned that the United Earth Government was not doing anything about the nebula but were instead focusing their military on the Tirolian Motherships instead. (Masters Saga: "The Hunters")
When Zor Prime destroyed the Protoculture Matrix, the Flowers of Life were allowed to bloom and spread throughout the planet. This was detected by the Sensor Nebula which sent its findings back to its masters. As a result, the Regess departed the known worlds of the Local Group in order to invade the Earth, thus bringing about the Third Robotech War. (Masters Saga: "Catastrophe," "The Invid Invasion," Robotech: Love Live Alive)
Behind the scenes
Robotech Art 3 makes mentions of Sensor Nebula as being used as a means by which the Invid discovered the worlds of Karbarra, Spheris, Haydon IV, Garuda, Praxis and Peryton due to the emanations made by the Flowers of Life.