There seems to be a crater nearby full of dust. If Spirit while bouncing around had hit the crater then it might have gotten stuck there. If so is there some chance it could have settled into the dust and been buried?

I guess I'm thinking of the Fall of Moondust Arthur C. Clarke story. If craters like this large enough were filled with this kind of dust could they explain the missing MPL and Beagle 2?

Also the bouncing around of Spirit would tend to make marks on the ground. Could this marks be examined, as they may have scraped off some of the exterior soil?

Now Spirit is on the ground there is the problem of finding subsurface materials. Here is a possible way around this for future missions. The lander could carry small bombs to be set off electronically. These could be carried to a place by the Rover and when they are in position the Rover is moved far enough away and the bomb is set off. It doesn't have to be a big bomb, perhaps more like a firecracker.

The debris on the ground has a good chance of being subsurface material then. Another way is to fling them out from the lander and explode them remotely. In that case there would have to be a way of ensuring it is far enough away, perhaps by being able to photograph where it landed.

This seems to get around the need for drilling into the ground which is much more technical and probably expensive to build. It would also allow some small seismic tests. The lander could measure the shock waves and build up a picure of what is underground.

Also larger rocks could be cracked open and the insides examined. There are plenty of explosives which should be safe to transport for this. As another example Thermite could be used, which burns at 3000 degrees. Maybe a small Thermite bomb could be put on some rocks and soil and it could be detected spectroscopically what materials come off it. Also it may be interesting in infra red how the heat is conducted.