Most likely the space is taken by Time Machine. It keeps making local snapshots when the persistent TM storage is not available. The local snapshots can pile up to as much as 80% of disk space. After the next backup, or when the disk space runs low, the local snapshots are automatically removed by macOS. So this is not really a problem that needs a fix, the system will take care of it.
That said, I'd agree that the System Information app needs a fix on High Sierra. It seems to include the local snapshots into "System" category and therefore it appears as used space, while Finder counts the snapshots as "purgeable", including it into the "available" space. This inconsistency is confusing.
DaisyDisk currently includes the snapshots into the used space (as opposed to Finder). Previously the snapshots could be revealed (./MobileBackups folder) when scanning "As Administrator", but starting from High Sierra and APFS, the local snapshots are saved outside the scannable area, so DaisyDisk simply shows a bigger "hidden space" segment. An update of DaisyDisk is said to be coming that will display purgeable space more consistently with Finder: https://daisydiskapp.com/manual/4/en/Topics/HiddenSpace.html?subtopic=StillHiddenSpace