A hidden compartment is read as a part of yourself not yet explored, or knowledge kept out of reach. Dream books often see its discovery as new insight rather than a threat.
When this appeared in your dream, what were you feeling?
My Cosmic Code has a private dream journal. Write a dream down and the app picks out the symbols in it, shows what dream books say about them and, over time, which words recur across your entries. The journal is stored on your device only.