Battery discharging whilst parked

Hello All
I have a 2016 94Rex with 20,500 miles, last week I received the dreaded "Battery Discharge whilst stationary" message, checked on here & immediately booked it into my BMW agent for replacement of the 12v battery. This is scheduled for next week.
On Thursday I parked the car with 99% battery level after charging it at home, this morning when I went to it, the battery level was only 51.5%.
Are these two phenomena linked, or are both of my batteries on the way out?
I hope not, because I love the damned thing, always makes me smile whenever I get behind the wheel.

Thanks all

So that is a week you left it?
If the 12v battery is failing to hold a charge, the main battery will come on to try and boost the 12v to "keep it alive".
If the 12v is not holding a charge, all that energy just gets turned into heat and the main battery will always be on, trying to charge the 12v.

I suspect that is why you got that message and also why the main battery drained that much over a week.

Thanks for the speedy response