While it is likely that Rumi Wakasa is just a red herring, if we assume that she is an agent of the black organisation I would say it is more likely that she is some agent who works under Vermouth. In the Asaka song case if I remember correctly Vermouth was disguised as Azusa, so seeing Conan there would perhaps have made her suspicious that Conan knows of the existence of rum and the kohji haneda case and to elicit a reaction from Conan this particular agent has been sent.