Space used in internal memory but nowhere to be found
I decided to keep looking for places that did not go reported by Windows (those reported by ES File explorer as "Other") and stumble upon a good candidate. In my case it was Google Maps.

Turns out I had a few "offline areas" downloaded on my phone from a recent trip and that was eating a full GB of internal space.

Deleting those maps fixed my problem.

I guess what I learn from this exercise is to always open "Setting" --> "Internal Storage" --> "Used Space" --> "Apps" to get to a screen called "Storage Used by Apps" that list the full set of applications on my phone and the amount of memory being used (from high to low). That should help me get started in pinpointing good app-candidates for "cleaning".