A manner of detecting the presence of alien devices, for example ONUs, that are transmitting upstream in an optical access network such as a PON. An alien detection device, for example a properly configured OLT, supports normal network operation and also monitors a selected performance characteristic. When the selected performance characteristic is evaluated to perform within satisfactory parameters, a protocol that includes a command to subscriber devices such as ONUs to go quiet, for example a rogue check, is executed and a signal detector monitored during the quiet period. An alarm may be generated if an alien device appears to be present.