well in fact Anchoring focus can leave the presence of the magician...
yep, you're right, they can leave the aura of the mage....
however, it's still an expensive trick to pull off! The mage has to be initiated, has to have the anchoring metamagic technique, then has to resist drain for the spell TWICE, THEN has to expend Karma for each spell they anchor equal to the force of the spell!!!
Not something I would do that often on my mage, especially considering how valuable Karma is to mages.....