Figure out Hidden used space in Linux

First Option:
root@devops-test:/ lsof | grep -i delete | sort –k8 | awk '{for(i=1;i<=6;i++){printf "%s ", $i}; print
$7/1048576 "MB" " "$8" "$9 }

Second Option:
Another situation was garbage file which was in /mnt/var/lib/mysql/garbage.file which again decreased disk usage to 23% upon removing the garbage.file
Tried to mount such files from root to /mnt using mount –bind / /mnt