Abstract: A system for detecting unintentional operation of a radio transmitter includes an antenna for receiving a r.f. signal and a detector for detecting the absence of presence of a component in the received signal that is indicative of a transmission from a radio transmitter keyed in an operating condition but not being used. In response to detection of such signal component, the detector produces a trigger signal. A delay timer times the duration of the trigger sigal and produces a delayed signal in response to the duration of the trigger signal exceeding a preset time period. An indication circuit produces an operator-alerting indication in response to receipt of the delayed signal informing the operator that the transmitter is unintentionally keyed in an "on" condition. The system also has a d.c. power source for supplying power to the detector, delay timer and indication circuit to operate the same.