Ah right my bad, misunderstood.
First, install DiskUsage anyway and browse aronud. Still check that you haven't got any nandroid backups on the device. They don't always get reported in storage. Move them to PC or cloud for safekeeping.
But it's likely your problem is similar to others I've seen in these forums. You're not the only one to experience this, also search a few similar threads "Nexus 7 missing storage" for instance here
When did you notice this, did this occur during an update to CM 10.2?
What bootloader version are you on (latest is 4.23 for Android 4.3)?
Reading around you may need to free up some space for a nandroid, completely unroot and reformat your data partition, reroot and restore.
Device: HTC Desire | Nexus 7
ROM: MildWild V-4.3 | Paranoid Android
Recovery: 4EXT Recovery Touch | TWRP
let's all try to get along ...but remember to search first...