Something may seem too irrelevant for a character to bother noticing, yet it is most relevant.
The brain screens out whatever seems irrelevant, and so the affected character can't notice it no matter how relevant it actually is. Examples are: Soulless Men from The Wheel of Time series, whom the Big Bad uses as assassins because no one would notice them entering or leaving; Alirea from the Twilight fanfic Luminosity, who could also murder you without you taking note of her approach or actions.
Hello, Unknown Troper. You'll need to get known to lend a hand here.