When he saw her reach for the weapon, Harry shifted his left arm slightly, letting his shield bracelet fall below the cuff of his coat. He didn't toss any energy into it yet, but began to slowly gather his will for it.
He snorted. "I'm no Watcher. I leave that to Giles. I'm a wizard. And there's a Hellmouth right behind you, trying to open. There's some complicated time energy wrapped around it, so I'm guessing - for whatever reason - it reached out and opened that portal to pull you through." A shrug. "I'm not complaining. If you know about Watchers and -- lurks, you call them? -- and the Hellmouths... and you clearly know what you're doing with that blade... I'm gonna make a wild leap here and guess you're a Slayer."
She knew too much otherwise. Harry was good at putting pieces together.