I went in the house for 10 minutes and when I went back outside the bags were gone. Later, my neighbor told me that a large white truck had stopped, and the driver took the bags and put them in the truck. I know that there was an electronics equipment recycling vendor in the neighborhood that day, and I suspect they may have taken them. There was no pickup tag left on the bags, and they were obviously not electronic equipment. They were not even close to the sidewalk. I called the vendor and they confirmed that they have large white trucks, and promised to look into it and call me back. They never did. I filed a police report, but just to document the incident.
Just be aware that any bagged items left in your driveway could be mistakenly taken. Probably a very rare incident, but I will certainly be more careful in the future.