Just bringing this point up in relation to the low cruising altitude question.
When asked why this flight didn't show on the usual tracking sites, FR24 replied the the transponder was either not working or "old" equipment. I take this to mean it was Mode C and thus not eligible to enter the airways system in Europe as a Mode S transponder is mandated.
Having recently replaced my old Mode C unit with a new Class 1 Mode S device (at under £2000) I wonder why this upgrade had not be auctioned in this case?