My disclaimer remains the same for all dreamers: what YOU decide it means IS what it means, for you.
IMO you are concerned that someone or something will disrupt your seeking the peace and help that playing the flute again will bring you. It appears that your grandmother has been a champion for you in the past, and maybe already does know about your depressive episodes or that you trust her to know at some point.
Is the friend that beat you up one of the two that know IRL? Perhaps that person doesn't share your idea of the path you have chosen, and wants to prove that it's wrong by telling everyone, including your grandmother? (by opening the case to show) This is a friend that doesn't fully support you.
What do you think now?