Chrome popup on random urls?
Using a bit of the scientific method, narrowing down the cause of the popup.
Other browsers didn't have the same issue, only Chrome. And the popup was occuring on my own NAS, with no other users reporting such an infection.
So, back to Chrome. I looked at my extensions. Over 2 dozen. Turned them all off, and the popups stopped. Turned them on, 4 at a time, until I found the one that caused this. When I clicked to look in the Chrome store, it's no longer listed. Mystery solved.
(Thank you, Allan, for the time and patience, and the SW reference, which helped me find unwelcome adware.)