A secret sub-neighborhood, also known as a "stealth neighborhood" or "stealth hood", is a special type of sub-neighborhood, first introduced in The Sims 2: Pets. Secret sub-neighborhoods primarily add new families in the family bin, new NPCs, townies, and secret lots. Some secret sub-neighborhoods can be visited, though the lots are not editable, and none of them can be lived in. Secret sub-neighborhoods do not appear in the sub-hood selection at the top of the screen, and are only referenced in the loading screen when playing a neighborhood that does not have all secret sub-hoods attached.
In The Sims 2: Pets, the secret sub-neighborhood is called Pets. It adds new Pet townies that are able to go into the main neighborhood and any other non-college sub-neighborhoods. It also adds the Roseland family and the Crittur family, both of which can be placed in the main neighborhood or any non-college sub-neighborhoods.
In The Sims 2: Seasons, the secret sub-neighborhood is called Weather. It adds the Garden Club Sims that are able to visit any lot in the main neighborhood and any other non-college sub-neighborhoods. The families it adds are the Ottomas family and the Ramaswami family, which can be placed into the base neighborhood or any non-college sub-neighborhoods.
In The Sims 2: Bon Voyage, the secret sub-neighborhood is called Exotic Destinations. It adds the "tourist" townies that appear at the vacation destinations. The family it includes is the Traveller family.
In The Sims 2: Freetime, the secret sub-neighborhood is called Hobbies. It includes many new secret lots, called Hobby lots, the Hobby instructors, and two new families, the Picaso family and the Cooke family. Unlike earlier secret sub-neighborhoods, lots in Hobbies can be visited. If a Sim has received a pass to a secret hobby lot Hobbies will appear as one of the available destinations when he or she travels to a community lot.
In The Sims 2: Apartment Life, the secret sub-neighborhood is called Magic. It adds two new lots, the Palace of Neverending Light and the Fortress of Eternal Darkness, new Witch Sims, many social group townies, and three new families, the Gavigan family, the Goodie family, and the Newson family.