There was something about the way Angel moved that had her utterly fascinated. He was able to disappear within mere seconds, leaving no trace of evidence that he had been there in the first place. He was able to be in one place at one time and then in the blink of an eye, he was gone.
She had been trained to notice things that most people wouldn't and yet still he seemed able to evade her. He was very able, in fact.
Mysterious did not begin to cover it and that he could disappear so easily both confused and concerned her.
If he could evade her, surely there were others who could too, whose intentions were much more dark and sinister. Of course she couldn't expect to win every battle she faced so that wasn't too weighing on her mind.
Since she had learned his secret, that he was a vampire, he was her biggest concern. If for some reason he turned evil, she'd have no chance against him. He'd find her, kill her and vanish without a trace left behind.
The graceful, inhuman movements that had her mesmerized would be her end.