It sounds weird, but I wonder if it's a tab loaded in the Chrome app?
Go into Chrome and swipe away all of the tabs. Maybe also go into the settings for the app and hit privacy and then tap the button "clear browser data".
It must be an app then. Until you figure it out, put the phone in silent (using the Motorola Assist app that comes with the phone maybe?) when you go to sleep so it doesn't wake you.
Mine did this once the other night.. Something about Walmart, then an annoying alarm that I've never heard..... Woke us up from sleep and I scrambled to silence it.. Nice....