Radio frequency signals have the potential to convey rich information about a node's motion and surroundings. Unfortunately, extracting such information is challenging, previously requiring accurate phase measurement, large antenna array structures, or extensive training. In this paper, we present LeakyTrack, a novel system that enables non-coherent and training-free motion sensing with a single antenna. The key idea is to create unique spectrally coded signals at different spatial directions so that geometric properties of the receiving node, as well as any potential objects in the environment, leave spectral footprints on the collected signal. To do so, we exploit a THz leaky-wave antenna and realize a color-coded scan in which signals with distinct spectral characteristics simultaneously emit across the angular domain. LeakyTrack infers nodal and environmental motion by analyzing the received spectral profile. We evaluate the performance of LeakyTrack via extensive over-the-air experiments.