It happens when it gets below freezing. It happened 3 times last Winter and I took it to the dealer and they replaced the bulb and it went away. Then it happened again and I took pictures since it would go off sometimes. So went back in months later and they said nothing was wrong with it so they gave me 5 free bulbs. lol Hasn't happened since. But it seems to only happen when it gets below 32 degrees.
They said it WILL be covered under warranty and I didn't pay either time and my car is CPO and they told me to bring it in if it happens again and they will take it apart but it hasn't.