General :: Disk Usage By Unknown File

i am facing problem of space usage by unknown file on /var partition.

[root@test var]# df -h

Filesystem Size Used Avail Use% Mounted on
/dev/sda2 20G 3.3G 16G 18% /
/dev/sda5 39G 36G 769M 98% /var
/dev/sda6 15G 4.7G 9.1G 34% /var/spool/squid
/dev/sda1 487M 16M 446M 4% /boot
tmpfs 1014M 0 1014M 0% /dev/shm

where as when I checked the using du -sh in its maximum utilization is about 5.9GB. i want to know the space being utilized by whom.

[root@test /]# du -sh /var
5.9G /var