How can I find the files being used by System? (where is my disk space going?)
My problem is related to the question What is the correct way to eject a APFS dmg from the command line?

What was going on is I was mounting APFS DMGs via hdiutil attach and only ejecting one of the disks. The disk left behind was taking up hard drive space. After I fully ejected the volume, my hard drive space came back.

The way I was able to spot this easily was by using Apple's Disk Utility application and saw a bunch of grayed out Disk Images.