Almost everything in modern BMW vehicles (and some others, as well) is controlled via computers and that includes the lights, fans, and nearly every module in the vehicle. When you shut down, the computer sends serial data messages out the CANBUS to tell they to go into sleep mode. If there's an issue with any of the modules, or the serial data bus to it, that device may not turn off, thinking it is still supposed to be working in the READY state. So, that can cause the drain on the battery to be higher than if it actually went to sleep.
So, it could be a wiring issue, a module issue, or a computer issue...IOW, it's sometimes hard to find. They can test the sleep mode battery drain, and if it's out of range, then start searching for the module(s) that are using that power. If the module itself is okay, then they need to follow the cable and inspect it and each connector, and that can mean dismantling things.
This isn't unique to BMW, though. For example, if you look at the wires going to the headlight switch assembly, there's a very small circuit board, and the wires going to it are like 24g...by no means big enough to switch the lighting circuit directly...all they need is power for the backlighting, and the circuit board, and serial data lines to talk to the computer to tell it where the switch position is. That's one reason why if, say, you left the rear wiper switch on, when you turn the car off then on again, even though the switch is still on, they won't work again until you turn the switch off then on again...the circuit announces changes, not the current state in most cases to keep the traffic on the bus down, and save power (there are exceptions to that).
It's a price we pay for the modern amenities and capabilities in our cars...they're harder to fix if something goes wrong. Diagnostics can only go so far...luckily, SS stuff tends to be pretty stable and long-lasting once you get through any infant mortality issues.
