The box knows if the MUX is active or not. On non-active transmitter channels the SNR is reported as zero. Bit Error Rate is the measurement of quality and not necessarily SNR.
However, I think that you are probably correct in what is being reported is actually junk and as a result the frequency finder is possibly next to useless on a ET10K.
As an experiment I inserted another 22dB of in-line attenuation at which point the picture was at the point of break-up. The SNR bar always still reported 100% and the BER never moved from zero. The SNR figure fell from 301dB (no additional attenuation) to 190dB (22dB of additional attenuation).
Selecting the first MUX it sees with a decent SNR or signal strength is a poor way of selecting the MUXs to scan. As mentioned previously both Crystal Palace and Bluebell Hill give me a strong signal and reliable reception on many days but because the path to CP is masked by nearby tall buildings signals from this source have proved unreliable longer term. You need reliable signals for unsupervised recordings.
For info.
I inserted a 4G filter in my system (approx 4dB insertion loss)
The frequency finder now finds 11 discrete MUXs rather than the previous 12
There are still the 3 missing MUXs from Bluebell Hill and now an additional MUX from Crystal Palace.
The filter is allowing Channel 56 to be seen from Bluebell Hill.
Code:https://www.blake-uk.com/home/534-f-type-external-masthead-lte-blocking-filter-35db-55db-upto-ch-57-proception.html