The material below is Open Game Content from a 3PP Source
Hidden Compartment (Ex): An eidolon has a concealed, built-in, watertight compartment in which items can be stored. The weight limit of the compartment is one-quarter of the eidolon’s light load, and the volume limit is 1 cubic foot (8 cubic feet for Large eidolons, 64 cubic feet for Huge eidolons). Spotting the compartment requires a Perception check with a DC equal to the eidolon’s HD + 15. Removing items from the eidolon against its will requires a steal maneuver. Removing an item with the eidolon’s cooperation is a full-round action.