Odor around air compressor

Pishta, my first thought was dead mouse. Worst smell in the world. However, they usually dry up and stop smelling after about 2 weeks.
Good point on natural gas lines. Are there nat gas utility lines in the area? A leak underground can travel strange paths. If there is natural gas service in the area I would call the utility company and ask if they could come out and check for leaks. Better for them to find it then you to find it the hard way.