Update: I pulled all the plastic shrouding around the battery and there was more oil. All together probably two or three cups of oil. I always carry a couple of spare quarts in the back but there is no evidence that there was any oil spilled on the carpet or anywhere along the sides and leaked down. It is all dry and clean. Unless the previous owner dumped oil down there by accident I can't come up with another logical source other than the air suspension compressor is leaking oil.