Final answer:
Mildred has a travel bag already packed when the firetruck arrives because she was planning to leave.
Step-by-step explanation:
Mildred has a travel bag already packed when the firetruck arrives because she is planning to leave. In the given text, it is mentioned that she quickly packs her suitcase with toiletries, clothing, lingerie, and dresses. She also writes a note explaining her reasons for going and leaves it on her pillow. This suggests that Mildred had already made up her mind to leave and was prepared for her departure when the firetruck arrived.