I had a 06, and an 09 as well. My 13 is the first to do this.
The dealer is at a loss for fixing it. The only successful repair I have seen is replacing the evaporator.

 

My dealer discovered that it was the evaporator. It was backordered for over a month and they got it installed today. The smell is gone now.

 

I just had the same issue with my 12 Eco with 17.xxx miles. They said they used some sort of cleaner on it and it seems to have gone away.
What the guys said about running the heat on high makes sense. Here I run the AC like 10-11 months a year not counting defrost and very seldom use the heat. I'll remember to turn it on and let it dry out once in awhile.

As for replacing the evap coil that doesn't make sense to me. I guess in theory if you had so much mold in it and can't get it clean but short of that its a waste. It's just a mini radiator.