Ctrl-Alt-Multiply is a known key shortcut. Someone just has to know it exists and think "I wonder what happens if I use this while the screen is locked". You

Ctrl-Alt-Multiply is a known key shortcut. Someone just has to know it exists and think "I wonder what happens if I use this while the screen is locked". You can get to that internal question many different ways, but its worth mentioning on top of that, security minded have trained their brains to be good at serendipitously coming up with "what happens if I do X and X" questions.