If you can’t see the driver's eye in his/her side mirror, the driver can’t see your eye either.
Work Step by Step
Light that travels from point A to point B retraces the path when going from point B to point A, even if reflection or refraction occurs. In other words, the laws of reflection and refraction are symmetrical with respect to light source and receiver. The sign is being a little cautious, because even if the driver can't see your face in the side mirror, there's a chance that s/he can see part of your vehicle in the mirror. However, better safe than sorry!