Dreams involving deceased loved ones, especially grandparents, can be quite profound and often carry significant emotional weight. Here are some possible interpretations of your dream:

1. Connection and Memory: Seeing your grandma in a dream, especially when you've never met her physically, may represent a longing for connection with your roots or family history. It could symbolize the wisdom and guidance that you seek from your ancestors.
2. Searching for Meaning: The treasure box could symbolize something valuable in your life—perhaps insights, memories, or unresolved issues related to your grandma or family. Your grandma's search for the key suggests a quest for understanding or closure regarding her legacy or teachings.
3. Fear of the Unknown: The voice warning you about opening the box reflects a fear of uncovering truths or memories that may be difficult to confront. It might indicate that there are aspects of your past or family history that you’re not ready to face yet.
4. Empowerment and Responsibility: Your decision to open the box and later inform your grandma about it suggests a sense of responsibility or empowerment. It indicates a willingness to take action and engage with your family's history, even if it is daunting.
5. Completion of a Cycle: Your grandma rushing back to the box may symbolize unfinished business or unresolved feelings. It could indicate that there are still matters to address in your family dynamic or within yourself regarding your heritage.

Overall, this dream may be encouraging you to explore your feelings about family, legacy, and the unknown aspects of your history. Consider reflecting on what the treasure box might represent to you personally, and how it relates to your life and experiences.