My wife just got an iPhone 7 and within a few days she told me the phone would suddenly show the ringer volume dropping as if she were pressing the volume down button. At first she thought it was the case she was using, but nothing seemed wrong there. Also, after taking the case off, I noticed at one point when the volume was all the way up, the volume down button did nothing.
I haven’t found anything about this online yet. I called Apple and they said bring the phone in for a replacement.