Clue

"hm?" Frank quickly averted his gaze as he caught a glimpse of the mask.

He wanted to join the faceless mercenary group because he wanted to keep his identity a secret so he knew how one would react if someone found out his identity after putting on the mask.

So he knew that he had to act like he didn't see anything else because it might bring doom upon him.

All of a sudden

"Woosh"

she grabbed the mask off of Frank's hand.

"Hey!" Frank looked at her with a look of confusion.

She had an ugly expression on her face as she kept a keen eye on frank

She was trying to discern whether or not he saw the faceless mask.

"sigh"

At the end, Frank didn't give off any hints of knowing what the mask meant; he just had a wronged expression on his face.

"I'm sorry," she said as she turned around to leave hurriedly.

Staring at her from the back, Frank skillfully maintained the expression of a confused person.