The increasing density of wireless communication systems has created new challenges for radio-frequency (RF) security monitoring in critical high security environments, particularly when low-level emissions are in the range of the background noise. This study presents an extended RF spectrum monitoring framework for the detection and analysis of unintended low-level electromagnetic emissions originated from electronic receiver devices. The physical detectability of a characteristic spectral component generated by a tested superheterodyne receiver was evaluated with two different approaches: 1) reference method for RF spectrum analysis: received-power and signal-to-noise ratio (SNR) measurements using a Rohde & Schwarz (R&S) FSH8 spectrum analyzer under different propagation conditions, 2) extended approach: comparison of the radiated RF component with corresponding spectral observations obtained from the protective-earth network, providing an independent conductive measurement domain for spectral cross-validation. Reference-based normalized spectral-similarity metrics were applied to evaluate spectral consistency, while repeated receiver-absent (H_0) and receiver-present (H_1) measurements were used for receiver operating characteristic (ROC) analysis. The effect of linear-power spectral aggregation was further evaluated using bootstrap-based uncertainty analysis with 2000 outer resamples. For the maximum normalized spectral-similarity detector, the direction-oriented area under the ROC curve 〖(S〗_maxAUC) representing detection efficiency increased from 0.6666 to 0.9369 for a single spectrum in 50 aggregated spectra in the 3 m configuration, while smaller or no corresponding aggregation gains were observed under the other investigated measurement configurations (5 and 7 m). The results demonstrate that the received power remains the fundamental physical detection parameter, while spectral similarity, independent cross-domain observation, and statistical validation provide complementary evidence for characterizing persistent low-level receiver-related emissions. The proposed approach provides an experimental basis for extending continuous RF monitoring systems with additional physically interpretable and statistically characterizable detection features for high security environments.