Secret Annex may mean:
Secret Annex of the Anne Frank House Tales from the Secret Annex, a collection writing Anne Frank published in 1949 Act of Seclusion, required by a secret annex to the Treaty of Westminster (1654)
See also Secret treaty Entente Cordiale, with a secret annex Molotov–Ribbentrop Pact, with a secret annex Treaty of Brest-Litovsk, with secret clauses