Free space discrepacy in BTRFS

I am missing about 40GB of free space on Ubuntu 18 / BTRFS system. Any idea what I can try? btrfs check /dev/sdb6 did not help:

root@Tablet4:/home/janbenes# du -xh --max-depth=1 / | sort -h
0   /cdrom
0   /media
0   /mnt
0   /srv
4,0K    /lib64
68K /snap
400K    /tmp
16M /sbin
17M /bin
48M /etc
148M    /root
219M    /boot
259M    /opt
1,1G    /lib
4,9G    /var
5,2G    /usr
131G    /VirtualMachines
143G    /

root@Tablet4:/home/janbenes# btrfs fi df /
Data, single: total=229.71GiB, used=183.17GiB
System, single: total=4.00MiB, used=48.00KiB
Metadata, single: total=2.01GiB, used=510.91MiB
GlobalReserve, single: total=512.00MiB, used=0.00B

Solution 1:

btdu is a tool I wrote which attempts to explain how space is used on a btrfs filesystem.

A common explanation is unreachable extents:

A feature unique to btdu is the ability to estimate the amount of space used by unreachable parts of extents, i.e. data in extents containing older versions of file content which has since been overwritten. This btrfs "dark matter" can be an easy to overlook space hog, which could be eliminated by rewriting or defragmentating affected files.