Radio signals are transmitted from a mobile detection unit such as a service vehicle in response to detection of signal egress from a cable distribution system such as a cable access television (CATV) system and coupled to the cable distribution system for upstream signaling by a resonant antenna such as a resonant loop or halo antenna and a sharply tuned filter to substantially eliminate other terrestrial or atmospheric signals being coupled to the cable distribution system. An optional attenuator may be included to limit signal energy in the upstream signal to prevent system damage. The mobile detection unit also preferably includes a global positioning system (GPS) receiver for determining position and time of any detection of signal egress or leaks and the transmitted radio signal preferably includes position and signal strength information. The radio signal is preferably transmitted in a burst having a duty cycle of 10% or less which may be multiplexed with signals from other mobile units preferably synchronized using the GPS time base.