... He scowled at the dark ceiling. Did they think he couldn't look after himself? He'd escaped Lord Voldemort three times; he wasn't completely useless....
Unbidden, the image of the beast in the shadows of Magnolia Crescent crossed his mind. What to do when you know the worst is coming....
“I'm not going to be murdered,' Harry said out loud.
'That's the spirit, dear,' said his mirror sleepily.”
I don't see a real mirror has been mentioned in the whole context. No one else was there and Harry was lying in his room alone. It might not be a literal mirror here. What does "his mirror" mean?
-- From Harry Potter - The prisoner of Azkaban Chapter 4, the Leaky Cauldron.