I have had this problem in my 09C4S and, many years ago, in an 86VW Jetta GLI. In the latter case it was the heater core and the smell was quite distinctive. In the former case it was/is humid air freezing against the windshield over night; the humidity being cause by the melting of snow that I dragged into the car on my boots and the relative airtightness of the 997.2. I am guessing that in your case it is the former but if you have a distinct sweet smell then think heater core.

 

It means Santa is in the vicinity to check on you!

 

Lmao thanks lacrosse back to the snow flake what does it all mean its there in the summer and winter santa cant be following me for 12months its just not right haha

 

This can happen just from weather changes when the temperature drops very rapidly and the interior of the car cannot adjust to the exterior quickly, but if it happens repeatedly you need to find the source of the moisture.