OS X 10.11.6 Missing over 200GB of disk space
Ok, I found it. Some time ago, I replaced my backup drive and the latest old backup from the previous drive seems to still reside on my disk. The old backup image was hiding under /Volumes/name-of-old-drive/. I could delete it safely with: sudo tmutil delete /Volumes/name-of-old-drive/Backups.backupdb/name-of-my-mac/YYYY-MM-DD-HHMMSS. 

To make sure that you do not delete any current backup, I recommend to temporary eject the currently used backup drive.

That was not easy to find, so I hope some other people will find this answer if experiencing similar problems.