Abstract: A smoke detector with a visual battery missing indicator and a removable cover includes first and second locking prongs molded in the detector and directed toward the cover. A battery is positionable between the first and second locking prongs deflecting same from a non-locking position to a locking position. In the locking position, free ends of the prongs can engage the cover, locking same to the detector. In the non-locking position, the prongs are not effective to engage the cover. If the detector is mounted on a ceiling, the non-engaged cover will not remain closed. In an alternate embodiment usable with a bracket mounted detector, a depressible blocking member is located adjacent a region where the detector is normally coupled to a portion of the bracket. In the presence of a battery this member is retracted, enabling the unit to be attached to the bracket. In the absence of a battery, this member is extended and the detector cannot be attached to the bracket.