Mine does it more often when it's cold, but since I garage the car it never really sits out and gets too cold... Well that and it's hasn't been super cold in the PacNW much this year. I first noticed in Dec of 2013.
I took it to the dealer 3 times and they could never reproduce it and neither can I. It just happens once or twice when I least expect it then not again for another couple weeks.
Maybe I should pay attention to my surroundings.. my car could be honking at other cars it likes.
