I have been noticing that my Moto G4 Plus had been switching to Vibrate mode or silent mode on it's own, just by leaving it idle, next to me. I have been missing calls right and left. I even performed the Factory Reset to see if that was it. I activated the System UI Tuner (pull down notification, press and hold the Gear settings symbol, then System UI appears towards end of Settings menu). I then went into the System UI Settings Tuner, under Do Not Disturb and turned off Volumes buttons shortcut (it was toggled on). So far, it had not been happening. I am hoping I have been pressing the volume down button by accident to turn there phone to Vibrate/silence. That's why I have having the buttons in the sides! My previous phones were the LG G2 & G3, buttons on the back, where I never accidently hit those buttons. I would constantly turn down my volume when I had my work Samsung Note 2.