How do they measure the exhaust? With the EM field in the cavity I would imagine precisely measuring the photon emissions would be very difficult and the SNR might not be high enough to draw a conclusion. If the force is generated by new massive particles it would be headline news in experimental physics as the most efficient energy-mass converter ever made but if it's somehow generated by high energy photons, what are the chances they have adequate imaging to measure UV, X, and gamma rays, especially if the emissions have some sort of complex interference pattern? (although the theoretical maximum for force/power input is much lower with photon pressure than the results in this paper so it's still unlikely)
I don't know that they are specifically looking for any exhaust other than RF emissions (which they say are not observed). If there were in fact an exhaust at some other frequency I don't know that it would be detected. (If such were present it would still be rather unexpected, in my view; there is nowhere near enough energy being pumped into the apparatus to start creating particle-antiparticle pairs, or to trigger any kind of process that would produce UV or X or gamma rays.)
