Turns out, I have that same thing.
In WinDirStat - Options, Show Unknown was OFF in mine.
Turn it on, and that same <Unknown> thing appears.
Here, the yellow thing at bottom left. Mine is only 12.6GB.
However, a subsequent run of WDS does NOT show it.
Notice the 'size' of data on the C drive (circled in red)
does not change by 12.6GB up or down.
From an older post at WDS:
blog.windirstat.net
What is “<Unknown>”?
"This mysterious item is just the difference between what Windows reports as the
free space on the volume minus
size of the files WDS can access. Please note the part
WDS can access! This is the important point here. WDS cannot access the files under System Volume Information on all the (NTFS?) drives, so it cannot sum up the sizes of these items. And by the way, we have had reports of up to 30 GB of “<Unknown>” space. "