My S5 active suddenly started showing full screen ads intermittently when I unlock the phone. I have not installed any apps recently, and there is no rogue administrator account on the phone. When I hit the left button to tile my running apps, it shows but no title is on the screen to identify where it is coming from. I've cleared my phone cache, tried Norton and app brain ad detector didn't tell me anything either. Android security scan came up with nothing. Any ideas?