iOS - Getting iPhone 6s plus storage full alert message
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.