Humans always seemed to have that reaction to his arrival. He didn't understand why, he'd informed her he was going to meet with her. He'd figured she would be expecting him. And they reacted similarly regardless of where he chose to appear - sometimes less favourably when he appeared directly before them.
His forehead creased and he then realized she didn't recognize him. "I am Castiel," he pointed out. "You agreed to meet with me."
As he'd already inferred, she wasn't terribly unlike Meg, but her reaction had been very much like something Dean would say. He didn't know if that boded well or not. He and Dean had clashed more often than not as of late and that was troubling in and of itself.