This is part of the story and therefore unmissable. The steps for this part of the story are:
From the translated letter in Mirror, Mirror trophy, you will have the phone number 0468-61-564. Find a phone to call it. It may be helpful to do this from the tobacco stand (1)
as you can then use a phone book and ask the woman at the stand for help too.
When you phone, you will be given the password:
Whatever the person says, respond with the answer after it from the list above. So if they say Mother's Earth, reply Comrades.
After the call, look at the phone book (If not at the tobacco stand, another phone book is found in your home, in the cupboard underneath the phone.
This will give you Amihama as a location. Speaking to the woman at the Tobacco stand will let you know it is located at the harbor. In the main street next to the Tobacco stand is the bus stop (2)
. Wait there for a bus and get on it to go to the harbor. As you walk into the Harbor area, the trophy will unlock.