This has happened to my wife's Droid Turbo (still on KitKat) twice now. The first time it was charging and we heard an Australian guy talking about home repair. No notifications, no controls. Killed nearly all running apps but couldn't find the culprit. This morning (a week after the first time) she received a call and throughout & after the call this same voice was doing home repair, loudly. I killed all the apps, nothing. I put the phone in airplane mode, nothing, he kept right on talking. This tells me that it has to be something on the phone (like PDG46 said) but I can't seem to locate it. Oh, I ran Avast and it found nothing. This is a strange one.