Storage Strangely Full My dad has a Moto G2, and I had similar problems with my Moto G2 and my Samsung Galaxy Y.

His storage is almost full - at 4.53 GB/5.51 GB. The strange stuff comes when we go to the storage section in settings, and it gives us a breakdown of what is taking up space.
Apps        1.99  GB
Images      132   MB
Other       20.38 MB
Cached Data 8.38  MB
(Nothing on anything else)

If I am correct - that's only ~2.15GB! Where is the other ~2.38GB?
And how do we get it back?

Moto G2 on Android 6.0 - System Updates says "Your device's software is up to date."
It is used as system storage ie the space on which your OS and system apps are installed. If you have expandable storage, you may be able to install user apps on an external sd card.