Short & Sweet:
* See all, hear all, say nothing.
* The truth is a secret.
* Unseen, unheard, untouched.
* Silence is golden, secrets are platinum.
* Always be two steps ahead.
Intriguing & Mysterious:
* Where shadows dance, secrets unfold.
* The world's a stage, and we're the unseen audience.
* Truth is a whisper, lies are a roar.
* The truth is out there, find it.
* We are the invisible hand.
Bold & Confident:
* We gather intelligence, we shape the world.
* The game is on, and we are the players.
* We are the eyes and ears of the world.
* No challenge is too great, no secret too deep.
* We are the masters of deception.
Humorous:
* We're not spies, we're just very good at blending in.
* Sometimes, the best way to get information is to ask politely.
* Secret agents: we do it for the thrills (and the paychecks).
* Our cover story? We're just average people, with exceptional skills.
* We're not always in the shadows, but we always know what's going on.
The best motto for your spy will depend on the specific character and the tone of your story.