Yea, but there's more than just secrets.... There's a room full of faces, and apparently Arya took one or more. When she did that to kill Mery Trant, they made it seem like it was a huge no no. But apparently they let her walk out with backpack full of faces. I think that needs more explanation.
See I didn't interpret it like that. Maybe I'm wrong... but I thought of it like this...
They have a room full of 1,000 or so faces. And whoever takes one must replace it with another. Like sorriority sisters sharing a closet of clothes or something.
But to get into the sorority you gotta buy a piece of clothing for to add to the communal closet, or in GoT terms you gotta bring us a face/kill someone.
So, when Arya didn't kill the lady she was supposed to to pay her entrance fee, they decided she forfeited her deposit and sent that bytch who didn't like her to collect her face to make up the difference.
I think the reason the OG no-faced Duncan McCloud looking dude let Arya go is not cause she killed her assassin, but instead because she used her assassin's face as payment to settle her debt to their club. I think if she had just killed her and tried to run she'd still have faceless ppl coming after her... but as of now, the many-faced god wanted a face and he got a face.
So she is clear with him.
And I don't think she stole that face from the hall of faces, I think she snagged that along the way.
Remember they had her cleaning and preparing faces for months... so she understands the process of how to snatch a bytch face and wear it from start to finish.