Dirty sock/garbage smell outside car

JT1991
I drive a 2008 Pontiac Grand Prix.
I live in North Dakota and we've had subzero temperatures lately. I often walk to work, so I leave my car parked in the garage for up to a week at times. Yesterday, I drove to work and it was left outside in the bitter cold for about 10 hours. When I got back home and parked my car in the garage, a little later I noticed a dirty sock/body odor/garbage smell in the garage. This morning, the smell was gone, but I went and drove my car for a few minute this afternoon and the smell returned shortly after parking it back in the garage. Now it doesn't smell INSIDE the car. The smell is just outside, and when I popped the hood up, there didn't seem to be a smell radiating from anywhere. I just smell it outside the car.
Any ideas of what could be causing this?
Thank you very much!