I know there isn't a worldwide black magic detector- that's why Harry's assertion that another Warden would definitely investigate and find Molly's handiwork seemed a bit bizzare.

There are never other Wardens poking around Chicago, checking on Harry's work.

You could handwave it by saying "Oh, the Gatekeeper knew there was black magic here, so he'd get suspicious if he didn't hear anything." But if the black magic just stopped, he'd probably just assume it had something to do with the Arctis Tor assault, or the Black Council (if he even knows about the BC, but he's pretty on the ball. He's either aware of it or on it)



It makes for a better story this way, so I'm not that annoyed by it. But it does seem like a bit of a plot hole. Remember, if Harry hadn't accidentally glanced at (what was his name? Nathan?) he wouldn't have even suspected Nathan had been mind-tampered.