I was having the exact same issue on my ext4 system and just wanted to post my solution for future reference. When my drive initially filled out, I deleted a bunch of logs out of /var/log. That cleared up a couple GB, but within a few days I ran out of space again, and du -h and "mount --bind / /mnt" did not point to the culprit. What finally got it was when I ran lsof.
lsof
...
rsyslogd 1766 root 2w REG 9,1 12672375940 264014 /var/log/messages (deleted)
...
When I had deleted the messages log file, the rsyslog service still kept it open, but hidden. Running a "touch /var/log/messages;service rsyslog restart" cleared up the problem and my disk space was reclaimed.
The lsof output can be a bit overwhelming, especially if you have a busy system (it was over 1000 lines long on mine). If you grep for "deleted" in the lsof output, it should help to pinpoint the problem process.
e2freefrag
anddumpe2fs
may be helpful.