I just experienced same Soc% loss over 24 hours period. The battery was charged to 50% (by wrong setting, should be 100%), car parked in garage.
Next day (24 hours), Soc loses 3.5% due to vehicle standby, 0% everywhere else. Mine is a M3 2023 with the 16V battery, not 12V.
The screen shows: "keep the car plugged in to avoid energy loss" !!!
Well keeping the car plugged in will of course consumes home electricity. From 3pm till midnight (peak period), the electricity rate in my area is high.
To avoid this high electricity rate, I can only keep it plugged in from 12am till 3pm !!!
Maybe it's best to turn off the circuit breaker that feed the Wall Connector during the peak hours.
Is this OK to leave the car plugged in with the circuit breaker OFF (at the panel) ?