Windows 10 C: running out of space rapidly

I started seeing warnings to free up disk space on my C: past week or so. My C: has 2TB of available SSD NVMe storage total.

1. I have done a couple of emergency relocations for Steam games and such like this, but it is still happening.
2. I tried looking into Google Chrome for temp files, maybe some of its data mining sharing policies and so forth, helped a little bit, still it is happening.
3. I tried running WinDirStat to see if I couldn't isolate where the issue might be contained, to no obvious avail. In fact, there is a mystery there, in visible (or invisible) files, there is not more than 500GB of obvious OS, programs, data, and so forth. Curious, when I check Show Unknown in the Options, a likely culprit appears, a 1.3+TBunknown file.

I wonder if possibly this is Windows 10 telling me I should run some pending updates, which there are. I run W10 Professional, and I like to police those, I have also not run them in a scant while, so I wonder if there are shadow copy snapshots going on that maybe shouldn't be. Still for the consumption that I am seeing, seems rather aggressive for W10 to be chewing up that much space so quickly if you ask me.

Is that what it is, possibly, System Restore being "too helpful" as it were? Any insights?