Dragging up this old thread, but he same is happening with my Dec 21 MIC M3. I also have a key fob. Happens very infrequently, maybe once every three months. This morning I got 3 short beeps when unlocking the car, another 3 when I got in, and another 3 as I exited the car.
No idea what’s causing it. Car was on 26% charge and one theory I have is that it maybe warning me of a low(ish) charge.