clv101
As with all crime, here are 2 strands to deaing with it.
You can detect it, the meaning of which is self evident
Or
You can prevent it. This means act, although you have no specific evidence that an offence is going to be committed, or because although there is specific inteligence or evidence, it is not suitable, or capable of being put before a Court.
The ideal is to achieve both those objectives, but sometimes that is impossible. So there are times that one of the 2 objectives is used, if that prevents an act of terror, or any other offence, then thats fine.
This is no different to any other form of crime, for example, information being recieved that can't be substantiated about a security van that is going to be attacked, and that attack prevented by the appearence of a police vehicle. Was there ever going to be an attack? Or did the Police presents prevent it? No one will ever know, the same applies with the incidents you mention.