I recently played the Curse of the Dark Pharoh expansion (only have played it once or twice before), there was an inner sanctum incounter that said to go through the exhibit deck and take the Triune Mask card. Couldn't find any card that was titled Triune Mask. Wasn't anything even close (though there is a Mask of the Three Fates, I guess that's kinda close in that Tri = Three). I know I haven't lost a card as I play in the same room and put everything back in it's own box afterward.

Could someone with the expansion please check to see if there is an actual Triune Mask card in the exhibit deck?

You are right, it is referring to the Mask of the Three Faiths.

here, check this out

http://www.arkhamhorrorwiki.com/Mask_of_the_Three_Fates

Card info

Type: Masks

Any: Exhaust to place tokens on this card equal to your current Sanity. Then choose one of these unused characters: Sister Mary, Mandy Thompson, or Carolyn Fern. You gain that character's unique ability while you have this item.

Upkeep: Do not refresh this card; instead, discard 1 token from it. If this card ever has no tokens on it, return it to the box.

Clarification

Some encounters refer to this card as the "Triune Mask."

FAQ says it's supposed to be the Three Fates.