There's no obvious reason for Nathan Stark to look briefly pleased by the challenge.

(Or possibly not by the challenge itself per se, but by the fact that it's SARAH that's challenging him, which is a subtle and crucially important distinction.)

Still, his pleasantly casual tone of voice doesn't alter.

"I'm not sure why you'd think that you should keep it a secret from me, given that it's my file you were trying to access."