Pressure does not mean you have air in the system. If you took the cap off while the engine was hot, then you should expect the coolant to rise, because you're releasing the pressure, and the coolant is replacing that volume in the expansion tank. BMW's cooling system is pressurized- it's supposed to be. The pressure increase raises the boiling point of the coolant, thereby keeping the engine cooler.
It's simple: if your heater isn't blowing hot air, then you probably have air in the system that's not supposed to be there. And always check the level when the engine is cold. Don't worry about what level it is when the engine is warm.