If you’re able, I would try using a program like WinDirStat to see what’s actually eating up your drive. If you do use WinDirStat, I do recommend running it as Administrator to be able to see all files as otherwise you may only see what files your user account has permission to see.
I truly don’t either, what makes you think this has anything to do with your TrueNAS? The timing of the issue?