We report on Ultra High Frequency (UHF) Radio Frequency Identification (RFID) phase measurements for the re-construction of passive RFID tag trajectories in an experimental setup. The setup consists of a UHF RFID reader connected to eight distributed antennas in a lab room. The antennas are multiplexed in time by phase-stable coaxial switches. The techniques of Gaussian processes are used to estimate missing phase values in the trajectory. The final tag's position in a two-dimensional space is estimated via an evaluation of the maximum phase difference in the trajectory, followed by a grid search to find the most probable position. For test measurements, this algorithm provides over a traveled distance of 15 m an Root-Mean-Square (RMS) error of approximately 50 cm.