Running Windirstat as Administrator will reveal most of this. In some cases it can be system protection files, corrupt files, old installers, or in my case, the Recycle Bin.

Per the Windirstat Propaganda Blog,

https://blog.windirstat.net/20061013/unknown-space/

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 “” space.

The root of the problem is permissions, apparently.