Interesting. I have to trust that both the dealer and the independent auto electrician will have thought of testing the alternator and it's associated control units but I will ask the question directly
Well my Boxster eat batteries for 7 or 8 years before the alternator started to fail. Clearly it was faulty all along.
Re your maths, yes that is correct, over 100 days at 30mA and about a month at 140mA. My Jimny draws about 40-50mA, quite a bit for a car with no alarm, most if not all must be the central locking. 50mA is a typical figure for a lot of cars, but if extra accessories, E.g. big stereo have been wired in they might be drawing current from the permanent live.
I also have a Ctek Battery Sense voltage monitor fitted, that way I can stand inside and check charge state.
www.ctek.com/uk/battery-chargers-12v-24v/ctx-battery-sense
Ctek say they are no longer made, but there are others.
Lets hope the garage finds something.
Robin