Missing disk space

So, I have following problem:

One month ago we saw that our linux server had drastic increase of disk usage: ~12GB in less than a minute time... Started at exactly 22:00. We had no cron job at that time...

This happened on 2 mount points: / (/dev/sda5) and /opt (/dev/sdb1) I'm trying to find what happened at that time, but no luck 'till now... Here some df, du infos:

df -h / Filesystem Size Used Avail Use% Mounted on /dev/mapper/opsi2-root 24G 20G 2,9G 88% /

du -hxs / 4,9G /

Also tried with find command, and I cannot find any files written in a given period :(

Could anyone provide any advice what am I missing here?

Thanks a lot