Every once in a blue moon when I pick up my locked iPhone (which auto-locks in just 30 seconds) and engage the home button just as the screen comes alive from the gyro sensing movement, it unlocks on its own. It just flashes the PIN dialog and slides right onto the home screen. I don't use Touch ID, and never stored my print with it even once to test the feature/hardware. It's been happening ever since iOS 11, with both my 1st gen. iPhone SE and my current iPhone 8.