I am curious how this happened. Last night I closed my (Dell Inspiron 15 3000) while Ubuntu was running. This morning I open the lid and was faced with the login / password screen which is normal and expected. Next I plugin in my USB hub that has mouse and keyboard plugged in. All was normal ( still on lock screen ).
Then I plugged in my monitor by plugging in HDMI cable. At that point the screen display went from the laptop screen to my desktop monitor and went right into Ubuntu without keyboard or mouse ever being touched. In other words it seemed to bypass security.