Listening for a lie out of both habit and natural instinct, Derek doesn't detect one, but he's not sure if he can trust his senses if she's not quite human. On the less negative side, she's got the weapons and has had the time to attack, and yet she hasn't.

"A Valkyrie." He sounds as if he doesn't entirely believe her, voice dry with a questioning eyebrow lifting. But then he's a werewolf and he's seen plenty of other things not to be too surprised if she turns out to be telling the truth. "As in Norse mythology?"