There's two things you all should know about BMW ac systems.
First is that like all automakers, they DO allow a "loss" or refrigerant each year. It usually between 15-30 grams. Which isn't much, but can add up after 5-10 years.
Second thing is that e-90s are prone to infinitesimally small leaks on the evaporator. You will NEVER find them at the same time it is being serviced.
If you lose charge over 2-3 years, the only way to fix it (assuming you don't have a large, obvious leak like a hole in the condenser via a rock) is to add tracer dye and recheck it months later. Generally, the dye will be found exiting the hvac unit through the ac condensate drain under the car if its the evaporator. There are no other common leaks on the e-90.
The evaporator is made by valeo
, and they make them for many automakers... Unfortunately, BMW is not the only company to have issues with them.
If they are leaking, it's usually so tiny that it literally takes years for enough loss to effect the system.
Also note that if it is leaking, you may be able to smell the odor of refrigerant oil when you first fire up the blower motor. It has an odd smell... Very "chemical" like.
Replacing the evaporator is quite expensive. The dash and all associated components must come out.
Many people opt to just evacuate ands recharge every 2 years to maintain system pressures. It's not perfect, but 200 bucks is better than 1800.