Len,
The 12v self discharging is intermittent. The first time was maybe four and a half years ago, just after we got the car s/h from an MB/SmartED main dealer. 12v battery was recharged and no problem thereafter. The second time was winter 2018/19. Car had been last used three or four days before. 12v again flat, HV System Failure on the display.
At this time my wife was in the hospital and I was more than a little distracted from car problems, but I did recharge the 12v again and used the ED every day, twice a day (indeed it was ideal vehicle vehicle for carving through city traffic) to visit her.
All well for a few weeks then a recurrence after a couple of days of non-use. From then on I was trickle charging every night and carried around a jump start pack and the car went back into regular use and I ceased trickle charging with no further problems.
I reported the 12v discharge issue to the dealer when I took the car in for annual service in October 2018, with about 50% SOC remaining. They read the stored fault codes, investigated further, and three days later pronounced both the 12v and HV batteries 'broken' and an unspecified module 'failed'. Over the next two or three weeks I watched (via the now RIP web portal) as the HV SOC gradually declined to zero. At this time I was unaware of the consequences of letting the 12v and the HV go completely flat. I guess the dealer was too.
I had the BAP and all the work and replacements were paid for by MB. I got the car back 8 months later (long wait for a new HV battery) and it has performed faultlessly for 11 months until two weeks ago, when I found the 12v dead once again. I've left it on trickle charge ever since, and the HV permanently connected to a charge point.
I'll be getting it back to the dealer ASAP but this time I have no BAP to rely on. Wondering if the dealer will offer some kind of warranty on the work done 11 months ago?
EDIT - I have the car booked for a diagnostic on 2nd July.