I was going to say use the key, but yours doesn't have a key lock like the E85
What Zoomer said, if you connect another battery via jump leads to the engine bay connections, it may work.
Can't believe BMW didn't put a pull release cable in for this type of incident :thumbsdown:
Edit
Just found this on Zpost
Leaving a regular battery inside the trunk doesn't seem like a good idea, batteries produce gases and some times leaks too.
As I understand that leaving the car closed is a big concern for you. Here's what I will do:
1. Close the car
2. Unplug the battery
3. Close the trunk.
When you get back:
1. Open the car with the key
2. Open the hood and plug your portable battery charger
3. Open the trunk using the button inside the car.
4. Plug the battery
5. Try to start the car
I'm not sure if the car will turn into transport mode, however you will probably have to take it to the dealer to erase the error codes produced by disconnecting the battery.