I have been driving 2012 A6 prestige for around 600 miles. Very often, in very low speed, when I press the brake to stop the car, I hear squeaking noise, then when I release the brake, I hear another squeaking noise. This problem happens more frequently in rain. Any idea why does this happen? The sales man explained that the car was still be break-in period, so that noise was normal. Should I believe that? Thanks.

Maybe you have some dust on your brakes?
Certainly, your pads must still be brand new.

I don't know. But I have a 2006 A6 Avant and just changed the brake pads about 1000 miles ago - when I brake, it's very quiet.

The noise problem is gone after 700 miles. Probably just because of break in.