There's caves in them thar hills
It certainly seems possible as no matter what image you look at that one hillside there is a big dark area. You would think that differing sunlight angles would show the hillside if it were not at least a shallow cave. It could just be that every satelite photo was taken at a time of sol where there was a shadow there though. Hopefully Spirit will continue to perform and find the answer to that question and many many others.