Hi gbull, that is a glitch on the left navcam lenses. We discussed this particular glitch in another thread. It always appears in the same spot on all left navcam images from Spirit.
or it could just be a dead pixel(s) which does not register any light. They would probably have the best equipment, so it is unlikely to be an equipment failure, but it could happen I suppose. On the other hand, it could be the sun glinting off a silver metallic disk-shaped object that is checking out the Earthmen's experiment.
The image below comes from Spirit pancam, sol424. A star like object can be seen that is definitely not an image glitch and I don't think its a star either...it could be something occuring naturally just as well as something of artificial origin, who knows? Anyway I'm convinced its "out there" and not a processing artifact.