Toward the end of Harry Potter and the Goblet of Fire, Barty Crouch Jr is forced to reveal his entire story, under the influence of Veritaserum. At one point, he tells everyone:
"My master sent me word of my father's escape. He told me to stop him at all costs. So I waited and watched. I used the map I had taken from Harry Potter. The map that had almost ruined everything."
The book doesn't explain this any further. How did Voldemort "send word" to Crouch Jr? And frankly, why aren't more people concerned that the Dark Lord is able to get messages to potential traitors within the highly-guarded and highly-protected fortress of Hogwarts?