The Casting Time description of Counterspell states.
1 reaction, which you take when you see a creature within 60 feet of you casting a spell
I am playing a spell-caster who almost always wears a mask. If he casts a spell requiring only a Verbal component he cannot be seen speaking the words for the spell. By my interpretation this means those specific spells cannot be Counterspelled.
I feel that the use of Metamagic Subtle Spell to avoid Counterspell corroborates this.
If casting without any somatic or verbal components allows someone to avoid Counterspell, surely casting in a way that those components aren't visible has the same effect because Counterspell specifies Sight as a requirement for the reaction.
Usually this is done by hiding behind cover, breaking line of sight to the enemy spellcaster.
Because Counterspell by wording and ruling seems to require Seeing the Verbal/Somatic/Material components of the spell, not the caster themselves, I think that localizing the cover to just the face (like a mask) should not trigger the reaction if that is the spells' only component.