Where is my storage? I have 12gb on my phone storage, but only half is accounted for. How can I reclaim my storage?

My phone is a OnePlus 1 running CyanogenMod. I get it that the extra space is taken by some system files or cache. The amount of space used by the system keeps going up (Half the storage space now) and I want to figure out a way to clean this. I tried several "cleaning" apps with no success.


Storage (click image to enlarge)

Update: disk usage shows me 4gb used by system, no details or options to delete. Any idea how to reclaim this space?
It doesn't usually add up properly.  Once you take into account system data, the OS, cached data (from apps, email clients, web browsing etc) it goes down pretty quickly.  Although it's not normally that far out!  What phone are you using?