FOUND IT!
I went through my list of devices and smart apps, removing many of them, one-by-one while watching the dnsmasq log. None of the smart apps made a difference. Nor did taking out a number of unused devices.
Not until I remove the Hue hub. Once I removed that all of the bogus DNS queries STOPPED. I’ve added the hub back again and the bogus DNS queries have not returned.
I also had this conversation, with more details, over in the smartthings subreddit over here:
So, why would the ST hub be using DNS to find a missing Hue hub?