i have seen it in E46 (318), E39 (530) and E53 (X5)

Depends on the year. On early cars the ac switch is either on or off. On latter cars there is a sensor that turns it on based on how hot it is inside the car and what temp you have selected for the AC to turn on. Also in the US all cars sold with automatic AC have to default to defrost so sometimes it can run through that cycle before it turns the AC on.
Last if the car has automatic AC and is turned off manually it will not turn on with the car.

MODEL AND YEAR Geeeeeesh NOT all bmws are the same when it comes to the HVAC,