JC,
The CCC (audio system controller/nav unit) is a complex head unit consisting of 5 different control modules. This is the "gateway" module for the MOST bus, which is a fiber-optic bus system that is used for the audio/communication systems. It was first introduced in the E65/66 7 series and has since trickled down into all new BMW products. The only car currently being made that doesn't have a MOST bus is the X3.
Anyways, the amplifier is on the MOST bus if you have the high-end audio system. BMW refers to the high-end audio systems (ie. logic 7 or the individual audio system in the M3) as "top Hifi". When you open the door, the MOST bus is most (no pun intended
) likely woken from sleep mode. I suppose it is designed this way so the CCC can begin to boot up before you actually start the car and drive off. When the MOST bus comes up, the amplifier is also powered up which could explain the sound you hear from the speakers.
So yes, the front tweeters are in a way connected to the Nav system. The speakers are hard-wired to the amp and the amp is connected to the CCC via the MOST bus. I think this explains what you are experiencing. Is it normal? That is a good question, but I have not noticed it in my car (and I have Nav & premium sound).
Is it something to be worried about? I don't think so, but it does show you the consequences of frequent locking/unlocking and the amperage draw that comes with disturbing sleep mode. This is one of the things the vehicle monitors to help diagnose battery charge issues (in addition to short trips, lights being left on, excessive storage time, etc.). Each time you open a door or hit the remote button, the bus systems wake up and it actually takes the car about an hour to completely enter sleep mode. Just something to think about if you have a kid at home who likes to play with your remote key.