iOS - Getting iPhone 6s plus storage full alert message Since yesterday, I'm getting a storage full alert message (not iCloud) on my iPhone 6s plus, iOS 9.2.1. 

Overall, my apps installed would take at most 10GB. 

Within Settings, the iPhone says it has 4.8 GB free. 
Inside of iTunes, it says 8.4 GB free. 

Also from iTunes, the Documents and Data section seems to take a lot of space (55% to 60% aprox.).

Has anyone have a solution around this?
I'm also having the same issue like you are. I was using the Public Beta version 4 of iOS 9.3 and that suddenly happened, it threw 710MB of free space on my phone into an unknown black-hole. I forced installed the iOS 9.2.1 (13D20 version) onto my iPhone 6 last night, freeing 16 GB from the phone. However I've lost them all again today with only 2.2 GB of free space left. The documents and data cannot be seen in the storage settings on my iPhone 6 at all. You're not alone on this one.