Hi snobbyformula,
Windows Hello face authentication utilizes a camera specially configured for near infrared (IR) imaging to authenticate and unlock Windows devices.
Surface Pro 3 has a front-facing infrared (IR) camera on your Surface, it recognizes your face and iris, allowing you to sign in to Windows 10 without having to touch the screen or keyboard.
When you tried to cover the camera of your Surface Pro 3, were you already on the Log-in screen? Infrared will recognize you in just a quick seconds, by the time the device loads up to the log-in screen. You may try restarting your device
and while booting, completely cover all cameras and let me know if Infrared still detects.
Regards.