tina8503,
My first inclination is your have a leaky caliber brake piston seal. You can easily check if that is the case by finding your brake master cylinder, open it, and see if you have a low fluid level. If you do, you probably have a leaky seal in the brake caliber piston. Just curious, but how many miles on your Bimmer? and when was the last time you had your brakes serviced? Brake caliber piston seal leaks are not something that would leak under normal wear and tear until the car has around 175-200K...or unless something has gotten into the brake fluid and damaged the piston seal. Since you say it has occurred on all wheels, i would suspect the master cylinder has been contaminated with brake dusts during some earlier serviceing on your brakes and when the brake lines were "bled" ...it got into the brake seals. Just my thoughts.
R/TxJohnSlaughter.