Thanks WJC and JW657--I removed the belt, and the sound DID turn out to be from one of the accessories or pulleys. It may be the idler pulley to the right of the water pump. It looks like my normally good skills at stereo-locating sounds, needs some training for work under a car's hood. This time, I used a cardboard tube from a used-up roll of paper towels as a cheap stethoscope (I know, I should just buy a cheap stethoscope), and when I got it close to this idler, the squeaking was loudest, and when I rested the tube on the alternator (actually my first suspicion), I heard no squeaking through the tube. I guess the squeaking might instead be from the water pump, but I'll deal with the idler first of course.
This might be the origin of the squeaking: A few weeks ago, I replaced the belt (it had snapped due to a frozen A/C compressor clutch/pulley). While doing that work, I rotated the apparently squeaky pulley I found today, and found that though it rotated pretty freely, it still seemed a little sticky to me, so I removed it and used some kind of spray lubricant (something better than WD-40, but I don't remember now) to flush out any grit from its bearing, and then I decided to put it back in place, instead of getting some grease into the bearing (though I don't remember if that idler's bearing allows you to get grease into it). Maybe my putting too thin a lubricant into that bearing is causing it to squeak, but I thought it was squeaking before I did this--maybe not. I normally take notes when I discover something on my car, but I can't find a note telling me when the squeaking started. I'll be removing the idler later today and seeing if I can get some grease into its bearing, and if not, I'll replace the idler.