There was a period early in Data's career where he would identify himself as an android immediately when meeting new people. He stopped after observing that biological lifeforms never identified themselves as such unless directly asked, and it has become such an ingrained subroutine he rarely thinks of it anymore.

He doesn't change his expression when she confesses to murder - at all, which is likely one of the slight tells she's noticing, his mannerisms lagging in realism compared to his physical features.

"I see. That would explain your incarceration on this ship." She appears to only be an older adolescent, and Data accesses his recently created database on the rights of children and the impact of trauma on their development. (He had not, in the end, needed to use it for its intended purpose.)

"Thank you for sharing this secret with me," Data says, "I appreciate your expression of trust in me as an authority figure. Would you like to talk about what happened?"

It's asked considerately, but with the same kind of consideration as he would use in asking her if she would prefer to sit on the left or right side of a bench.