The objective of the paper is the estimation of the close motion of a UHF RFID tag with respect to a standing one by resorting to the measurement of the Received Signal Strength Indicator (RSSI) of the RFID signal backscattered by one of the two tags. Exploiting the electromagnetic coupling between the antennas of the two tags which changes according to their distance, an Extended Kalman Filter is designed to solve the problem. To improve the quality of the estimation, the EKF also includes the estimation of the relative velocity between the tags: this also allows in some cases to solve the ambiguity characterizing the RSSI measurements which have a not unique dependence on the distance. The proposed setup can be applied in several contexts, e.g. robotics, manufacturing, safety and in any situation where the variable distance between two close objects should be monitored.