Going back to the mirror signaling: a mirror is meant to reflect light, not scatter it. Therefore, the sunlight that was reflected off of each of Ben's and the Others' mirrors would stay in a consistent direction. Since they were only rotating them in an up and down motion (along a horizontal axis), only people standing in the path of the width of both those mirrors would be able to see the reflection. This is exactly why it's a good communications tool because of its relative exclusivity with visibility.

This is the same phenomenon when someone reflects sunlight with their watch face into your line of sight. You don't even notice the reflection at all until the beam of light is directly in your eye.

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Abaddon