I went through every menu on the camera, and couldn't find anything amiss. I was beginning to think the camera had developed a problem with the exposure mechanism -- after all, it gets some rough use, and the climate here, with salt air and sand and red dirt, isn't particularly kind to electronic devices.
Finally I looked at the front of the lens. Then I looked at it again, with a bright flashlight, just to make sure of what I was seeing. There, on the front glass, was the biggest, blurriest, smeariest fingerprint I've ever seen on a camera lens! It had the same diffusion effect that photographers try to achieve by smearing vaseline on a clear filter. Only, in this case, just off center in the image!
It cleaned off okay, I think, with some isopropyl and a q-tip, so I was relieved.
But, shouldn't checking the front of the lens have been the FIRST thing I thought of? For that matter, shouldn't I check the lens each time I pull out the camera?