In a dangerous conditions sensing alarm system which includes a plurality of interconnected dangerous condition sensing units (Unit A, Unit B 70, Unit C 71, Unit D 72, Unit E 73), such as smoke detectors, of the general type in which each unit responds to an apparently sensed dangerous condition by issuing an audible alarm and also by transmitting, from an input/output terminal (20 - I/O), a signal to the other units (received as an external alarm signal at corresponding input/output terminals of the other units) so that an alarm condition sensed by any one of the units results in alarms being sounded by all the units, a unit initiating an alarm (which may be false) is marked by an alarm origination circuit. The alarm origination circuit includes a visual indicator such as an LED (61) and a latch device such as an SCR 51 connected in-circuit such that, when the SCR is turned 'on', the LED is activated and remains lit until the SCR is turned 'off'. The SCR is turned 'on' by a signal developed from the audio frequency drive to the unit annunciator (22) unless this action has been earlier inhibited by an electronic switch (52) which, just before the local alarm is sounded, places a virtual ground on the SCR gate (75). The electronic switch is activated by an external alarm signal more quickly than the local unit's audio frequency drive is established. Thus, an external alarm signal will not cause the LED to light at the local unit.