Billie... isn't jumpy, actually. Someone suddenly appearing is a concept she's familiar with — the Sea Wolves all knew the same teleport spell, so this happened fairly often — and that part doesn't bother her. No, what she reacts to is someone being far too cheerful right up by her ear as she's in the middle of pulling her whaler's mask off.

Yeah, she can tell it's Strange by voice alone. That just means he doesn't get punched. Instead, she gets the mask off and immediately turns to put a hand in the mage's face and shove him away.

"Never do that again," she tells him, firmly, like she's scolding an idiot who's just learned to do that. Which is, in fact, what it is. "Unless it's to put a knife in someone's back, which I'm pretty sure you don't do."