He raises an eyebrow that the comment that he's like her; it's not just the arm then. Moreover, the implication that she didn't have much of a say in it. He's a bit curious - if it's more than just the surprise gun, she's far too confident in her abilities (and, more to the point, good at hiding it,) for it to be a recent thing, and Milly already looks pretty young.

Contrary to popular belief, though, Raiden does have some tact and a few social graces, so he knows better than to ask her about her own situation. Especially this early into a conversation. Instead he gives some thought to the best way to answer her questions - trying to find the best balance between 'honest' and 'avoiding the issue' so that it doesn't seem as though he's dodging as much of the topic as he actually is. (Being detained in a TRIDENT facility for a year just isn't a topic he wants to broach on a first conversation with a stranger, is all. Or anyone for that matter.)

"I've been back about three months." Obscure it with the truth, brilliant. Expert spy tactics. Snake would be proud.