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."