Mages walk the edge of what normal people consider reality. Because of their magical nature, they sometimes escape the notice of Sleepers. Their very existence is an anomaly, and some of them just evade notice. This effect manifests differently for different mages.
Although the Arcane Background doesn't make mages invisible, it makes them less noteworthy. An Arcane mage seems nondescript and not particularly noticeable. Features just seen to slip away from memory, and the mage just never seems to get caught on film. Records disappear, people forget the mage's name of even assume that discussions are about someone different, and witnesses can't garner more than "That guy. Girl. Whatever." The mage doesn't trigger these effects actively; they just happen. The mage can, however, consciously dampen the effect and allow others to see her as she truly is.
You add your character's Arcane score to any Stealth rolls you make, and your opponents reduce their Perception or Investigation dice pools by a number of dice equal to your score in Arcane. Note that Arcane only helps when the mage is inconspicuous or absent; if the character is screaming, waving around a sword or otherwise drawing attention to herself, Arcane doesn't help. Of course, people might give conflicting descriptions later or be hard-pressed to remember her name. When your character is directly involved in combat, this Trait gives her no benefits.
Note also that a character with specific, extremely unusual traits - like purple hair, a peg leg or huge size (insert browwaggle here) - will not be able to conceal those traits; they stand out too much in people's minds. Still, "that one-armed guy with... uh... hair... of some color" is a start.
x You're just as noticeable as anybody else.
• You blend in with the crowd.
•• You're easy to forget.
••• You're difficult to follow.
•••• There are scant photos, papers or records of you, and people can't even agree on what you look like.
••••• In other people's minds, you don't even exist.