The cover of a ceiling mounted smoke detector is hung at one side by a chain from the detector frame. At the other side the cover has a clip which engages the smoke detector battery. Thus if the battery is not in place the cover hangs down from the chain providing a signal indicative of the absence of the battery. A flag is provided which is pivotally mounted inside the cover and movable on the pivot through an opening in the bottom of the cover. A magnet attached to the flag magnetically engages the diaphragm of the horn alarm in the smoke detector and maintains the flag at a raised position within the cover. When the horn alarm is sounded, the vibration of the diaphragm causes disengagement of the magnet whereupon the flag pivots downwardly to a position at which it extends below the cover. This provides a visual indication that the horn alarm has been sounded.