Check out this article on the BBC News site.
But the fact is they had concerns that there may have been somebody carrying sensitive material that may have directly or indirectly undermined our national security. And I'm glad the police took the action they did.
That's my emphasis, but surely "may directly or indirectly" covers just about every single possibility? In other words, they can stop who they want, when they want, for whatever reason they want. Even for no reason at all.
Welcome to 1984.