Yea I'm curious if this would actually be propane, or just your septic tanks vent stack. Have you tried climbing up and taking a little sniff at the vent stack just to see if it smells the same to you? Foggy mornings will hold smells like this. I have always noticed here in Florida, in the winter, smells are so much stronger in the winter, and it is because of how the temperature drops at night and we have more dew and more fog. I feel like I can smell fires for miles in the winter. I would try to verify you are smelling propane. If you feel it is definitely propane I would remove all propane tanks from my property and see if the smell persists.