I use the CALIBRE_TEMP_DIR environment variable to give calibre a dedicated location for is temporary files (C:\_Calibre\Temporary). Occasionally something will be left behind, often as not an empty folder, rarely even megabytes never gigabytes.
Why did I do that? A year or two ago I was getting a quite a few small calibre_xxxxxx folders left behind in the standard TEMP location - mainly from conversion.
I have one library that's almost 700GB, but I don't do conversions or ebook-editing on that library, its just a repository.
Try giving calibre a private temp folder - at least you'll be able to monitor it closer.
BR
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