Improved Invisibility and the Concealment power are your friends. A mage really should be undetected by the "muggles" until you open up.
I finally got to try that last session
Seconding the idea of not tipping your hand too soon. Backing up the stealthy spellwork (including noise suppression) with actual stealth skills doesn't hurt. It
really doesn't hurt to have a good codeslinger around to back up the boltlobber, as technomancers are even better targets for "recruitment". It's in everyone's best interest to suborn the opposing force's sensors, anyway.
With regard to perception rules, if I'm over-casting an area debuff like Chaotic World or Orgy, I really, really want the combatants Over There to be paying all their attention to that magic in the air around them ... and not to me or any other threats on my team. If I read the description right, CW should be getting their targeting system's attention too.
On the other hand, no one pays attention to the kid on a skateboard rolling by (light armor behind the pleather jacket, high agility, and a pistol and switchblade just because) or the drunk slumped against a wall (if you don't plan on moving around, why not wear more armor?) or the sniper nest ... that their drones maybe would have seen if they hadn't been hacked already.
As a 'teachable moment', take the time to show your team's mage how his friends "handle" an opposing spell-jockey in combat. After the composure check,

the value of camouflage, concealment, cover, disguise, deception, and holdouts should become self-evident matters of self-preservation.