If Giles had been there he'd have shouted at her for not noticing the new arrival until he was right there. Then again, Giles would have shouted at her for getting into this situation in the first place.
As it was she looked up, taking a moment before she recognized him.
"Hey Sam," she tried to grin before looking around.
"You're kinda late to the party," she couldn't help laughing at that before she noticed the gun.