I think there was a thread similar to this before.
http://f10.5post.com/forums/showthread.php?t=654278
"The sensor looks at Infra red light, not visible light.
1) Sun low on the horizon. Even with blue skies the lights may turn on.
2) Some water repellent coatings out on the windscreen. Dont put them over the sensors.
3) Dirt over the sensors. Keep the area clean."