Thanks for the post; it's good to know how frequently this problem is occurring.
I per-ordered the iPhone 6S and received it Sept. 25, 2015. Just 4 months later (Jan. 16, 2016), I took the iPhone into the Apple Store Genius bar because it had 'phantom vibrations' in the absence of actual notifications, which turned out to be a problem with the vibrate/ring toggle switch. The phone was replaced.
Now the problem is beginning to happen again, only three months later, on the replacement phone. How unlucky! We'll see what the Apple Genius says this time.