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...