Recent studies figure out that hidden eavesdroppers can be detected through RF (radio frequency) leakage from their local oscillators. Inspired by this finding, this letter presents Phantom Eavesdropping, an emerging wireless signal eavesdropping technique which renders hidden eavesdroppers immune to the RF leakage-based detection to further conceal their presence. At its heart is a dynamic alteration process regarding the local oscillator's oscillation frequency. As a result, the RF leakage from the local oscillator is whitened in the frequency domain and thus the corresponding eavesdropper becomes footprint-less. Practical experiment shows that Phantom Eavesdropping does not affect normal Wi-Fi eavesdropping while further rendering eavesdroppers transparent to eavesdropper detectors.