So, PC with both STILL and SILENT meta magic feats casts such a spell that does not have Material Components - or also has the Eschew Materials Feat.
Would this provoke an AOO?
If the spell had no visual effect, would anyone even know a spell had been cast? IF the spell was a Charm or Suggestion and the target failed their Save, would any observer know?
you provoke while casting a spell, when you take on that "far away look" and concentrate on casting rather than defending: your opponent spots the distraction and slashes in...
Casting defensively has you actively share that concentration between casting & maintaining awareness & defence - at the cost of potentially losing the spell.
Still & Silent may conceal what you are doing, but you still concentrate and get distracted - wham!