With thanks to Wikipedia: -
12K/.dbus
17M/sbin
78M/etc
0/proc
1.1G/tmp
4.6G/home
0/misc
4.4G/usr
5.2G/var
346M/lib
16K/lost+found
0/dev
72M/root
4.0K/cgroup
6.5K/mnt
7.3G/opt
4.0K/srv
4.0K/boot
4.0K/media
4.0K/selinux
30M/lib64
0/sys
9.1M/bin
0/net
23G/
23Gtotal
$ du --max-depth=1 -c -h -x /
12K/.dbus
17M/sbin
78M/etc
0/proc
1.1G/tmp
4.6G/home
0/misc
4.4G/usr
5.2G/var
346M/lib
16K/lost+found
0/dev
72M/root
4.0K/cgroup
6.5K/mnt
7.3G/opt
4.0K/srv
4.0K/boot
4.0K/media
4.0K/selinux
30M/lib64
0/sys
9.1M/bin
0/net
23G/
23Gtotal
Now I can see the villain :-)