Yes it is a metaphor because he does not literally have a key to her heart. It is just figurative speaking.
Answer:
As the chapter ends, what is Scout thinking about? ... She finds herself thinking of the Radley house and Boo's imprisonment in it. She asks Dill, "Why do you reckon Boo Radley's never run off?" Dill's answer is that maybe Boo "doesn't have anywhere to run off to
Explanation: