my 6s, 16gb, went from 6+ gb to 792mb, with no new apps, photos, etc. only thing I did was connect to iTunes before that. Do I need to restore? On 9.3.1. Thanks.
It was a movie, did the "download movie in itunes" to clear storage space trick, but there was room to download movie. Found movie in videos, deleted, back to normal. Thanks.
While the others are correct, something likely synced to your phone you were unaware of, photos, movies, etc.
Be aware there appears to be a bug that sucks up huge amounts of space and can lock up gigabytes worth of storage without explanation. I just Fred up over a GB of space on my iPad using this method.