Not as yet, but I am going to give it a bit more effort now that 5.1 is in.
EDIT: Turned out to be fairly easy. I did a new verify, then went through My Games/Galciv3, Program Fiies/Stardock/Galactic Civilizations, and ...common/Galactic Civilizations III.
The only directory with a new timestamp was ...common/Galactic Civilizations III/data
Looking in this directory, there was exactly one file with a new time stamp: GC3.bin.
So that is the file that is causing the issue.
--------> ...common/Galactic Civilizations III/data/GC3.bin
I immediately revalidated the cache and no issues were found. (It did show a "downloading 0 of 1 files" at the bottom of the screen while validating however).
I started the game only to the initial menu, then exited.
Then I checked and guess what...GC3.bin had a new time stamp, in spite of the fact that the program said there were no issues.
Wonder if I should reinstall Steam. Pretty drastic, but whatever.
Well, I can more easily do a complete reinstall of GC3.
This morning I carefully removed everything from My Games, Program Data, and the Steam directory for GalCiv3 and used the Steam function to delete local files. Then I reinstalled the game (with optin patch). The behavior was the same. After installation and running the game to the opening menu, I exited and validated the files and it found 1 to download and it was GC3.bin, based on the time stamp.