Continuity mistake: After Jared and Mallory visit Lucinda, back at the house when Mallory takes Jared outside to show that the goblins are gone, she is not using the seeing stone. Then she reacts when Hogsqueal approaches as if she can see him but in the next shot she acts like she can't see him.
Suggested correction: Actually she's reacting to hearing him, not seeing him, which is why she asks who Jared is talking to.