An audio indicator apparatus for locating boundaries of optically divided detection zones covered by a passive infrared detection system having a collector for collecting infrared rays from the zones of an area to be monitored, and a detector for passively detecting infrared rays collected by the collector and for generating a first signal having an amplitude responsive to the rays collected by the collector, which includes a signal generator to be connected to a speaker for producing a second signal responsive to the first signal, the second signal being indicative of activity of the detection system at a point, within the area to be monitored, where an infrared radiation emitting object is moved, to locate the boundaries of the zones.