Localization of sensor nodes has become a fundamental requirement for many applications over Wireless Sensor Networks (WSNs). In this paper, we propose a range-free approach based on the Received Signal Strength (RSS) changes. A discrete and probabilistic localization method is offered to localize a moving beacon relative to sensor nodes. In this method we came to a more reliable and accurate result without increasing the number of sectors. We achieved a considerable improvement in localization reliability by defining the primary and secondary sectors and utilizing a Kalman approach for making decisions. The decision making process is based on the allocated probabilities given to each of these sectors. Simulation results demonstrate an average accuracy of 96.3%.