Who stole my disk space ?

In my Cube running 10.3.9 I have a 19 GB harddisk. It's currently 98% full with 18.6 GB occupied.

oswald:/usr/bin root# df -k
Filesystem              1K-blocks     Used  Avail Capacity  Mounted on
/dev/disk0s3             19908432 19276288 433060    98%    /

but when I run du -sh on / (as root) to check where the space is used

all this adds up to approximately 12GB. So where is the rest ?

Peculiarily the Finder and du differ by 110 MB on my Library folder. Is this because the other files are owned by other users ?

It could be that after deleting a ZIP file of a mere 6 GB, using the Finder, something might have broke inside HFS+. Or maybe the log of the journalling filesystem was not properly cleaned up ?
Another possibility is, that my box has been opened up and that a root kit was installed. But when I looked at the WeaponX source code, I thought, it should be fairly easy to trick a root kit, based on that code, to reveal itself. I tried, but I didn't find anything.

Solution ? I rebooted. Mac OS X cleaned itself up, I suspect it was the log:

oswald:~ nat$ df
Filesystem              512-blocks     Used    Avail Capacity  Mounted on
/dev/disk0s3              39816864 25264232 14154464    64%    /