Low-Z injections into runaway electron (RE) beams have been shown to result in a benign termination of the beam, without significant heat loads and damage to plasma facing components [^1]. Following the low-Z injection, the improved thermal conduction to the wall by the neutral gas causes the cold companion plasma that coexists with the RE beam to recombine. Experimental evidence suggests that the composition of the companion plasma and its recombination are key features of the benign termination. Knowledge of its power balance is therefore essential to understand the operational limits of the benign termination scenario and how it can be extrapolated to ITER. An integral power loss mechanism of the companion plasma is through electromagnetic radiation. However, following massive gas injections for the benign termination the radiated power increased to several megawatts as measured by bolometers of JET. We show through a collisional radiative model that these measurements are inconsistent with a cold thermal plasma. To identify the origin of this radiation, the diagnostics overlapping the spectral coverage of the bolometers were studied. The visible cameras and UV diamond detectors exhibited an increase in their signal following the gas injection. However, only bolometry revealed comparable levels of “measured radiation” as during RE experiments when injecting gas into the empty vacuum vessel. This confirmed that the presence of the gas caused false measurements on the resistive bolometers used on JET. We employ a heat conduction model [^2] to demonstrate that geometrical features can affect the calibrated cooling time differently for the measurement and reference absorbers making up each bolometer channel. Through Joule heating, an imbalance in the bridge is induced and erroneously interpreted as radiated power. We show that this effect can represent the majority of the radiated power measured following low-Z injections into RE beams. Simulation results with the code DREAM [^3] for the power balance of the companion plasma following Low-Z injections are compared to experimental results on JET. [^1]: C.Reux et al., [PRL 126 175001 (2021)](https://link.aps.org/doi/10.1103/PhysRevLett.126.175001) [^2]: H. Meister et al., submitted to PPCF (2024) [^3]: M. Hoppe et al., [Comp. Phys. Comm. 268 108098 (2021)](https://www.sciencedirect.com/science/article/pii/S0010465521002101?via%3Dihub)