Why am I reminded of that episode of The Simpsons where "Bear Patrol" was put into effect.
Later, a full-force Bear Patrol is on watch.  Homer watches proudly.
Homer: Not a bear in sight.  The Bear Patrol must be working like a 
       charm.
 Lisa: That's spacious reasoning, Dad.
Homer: Thank you, dear.
 Lisa: By your logic I could claim that this rock keeps tigers away.
Homer: Oh, how does it work?
 Lisa: It doesn't work.
Homer: Uh-huh.
 Lisa: It's just a stupid rock.
Homer: Uh-huh.
 Lisa: But I don't see any tigers around, do you?
        [Homer thinks of this, then pulls out some money]
Homer: Lisa, I want to buy your rock.
        [Lisa refuses at first, then takes the exchange]
The absence of something doesn't mean something is working, there wasn't any attack on American main land soil by Al Qaeda after the 1993 WTC bombing until 2001.
-SW