Thanks, Rob, you're always a great help and I appreciate it.
By "operating folder" I mean the folder where my project is stored. The .seal and all image files are kept there. I specify it the first time I Save Project.
I looked at my Global Preferences with my project open and the Swap Storage Directories shows only one: a Temp folder in my App Data / Local path (which I believe is the default). I don't see a path to the current project folder anywhere in the Preferences.
I'm attaching a snapshot of my current project folder, with the mystery files building up, if that helps. It looks like maybe the mystery files are written when PI closes, judging by the file times from yesterday.
As an experiment, I moved all the mystery files (only) to a temp folder, started PI, and loaded the project. It showed this error stream on the console:
Reading image: Image_a
*** Error: Parsing ImageWindow MainView child element (line=12264 offset=6): No such file: D:/gb/Astro Image Processing/NGC6946 Fireworks Gal/Image_2.data/ZGUCXV1ZM-000001
* Loading images
Reading image: bkgd1
*** Error: Parsing ImageWindow MainView child element (line=44871 offset=6): No such file: D:/gb/Astro Image Processing/NGC6946 Fireworks Gal/Image_2.data/ZGUCXV1ZM-000008
* Loading images
Reading image: range_mask
*** Error: Parsing ImageWindow MainView child element (line=44954 offset=6): No such file: D:/gb/Astro Image Processing/NGC6946 Fireworks Gal/Image_2.data/ZGUCXV1ZM-000009
* Loading interface states
* Loading embedded XPSM
Reading icon: Process01
Reading icon: Process02
* Regenerating masking relations
* Updating workspaces
** Warning: The project has been loaded partially with 3 error(s).
1.209 s
The .xisf image files don't even load without the mystery files, so some of them at least seem to be critical.