How can I find the files being used by System? (where is my disk space going?) If I go to About This Mac and click on the Storage tab, it tells me that I have 500 GB of Flash Storage and that 15.49 GB of 399.57 GB is free. 

It will also tell me that 226 GB is for System, 130 GB is for Documents, and 25.8 GB is for Apps.

If I do: sudo du -hx -d 0 /*

 19G    Applications
2.5M    bin
 15K    dev
4.0K    etc
1.0K    home
1.9G    Library
1.0K    net
1.0K    Network
 16G    private
1.1M    sbin
6.6G    System
4.0K    tmp
157G    Users
848M    usr
4.0K    var
 36K    Volumes


Tools like OmniDiskSweeper or Disk Inventory X will report the same basic information. They will generally account for the space that the Storage tab says is being used by Documents and Applications, but I cannot seem to figure out where the 226 GB of files are that the "System" part is using. I had this situation come up once before and rebooting the machine cleared out the used space and I got a couple hundred GB back. I could do that again, but I would like to be able to track down what these files are. 

Any suggestions?
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.