It might be a 3rd party app that's causing this. I cleared my app cache and haven't experienced it again yet.
Another forum suggested wiping the cache partition, but I haven't gone that far yet.
Starting today, my 6P is altering my notification volume settings. I keep setting it to vibrate, but it keeps returning it to a low-volume notification sound.
It's not in my pocket--it's sitting on my desk next to me.