That is how everything starts,for me too.Modern BMWs work at coolant temperatures up to 230F,and to keep the coolant from boiling the pressure is up to 20 lbs.There is no small leak in a system with a 20 lbs of pressure.Every small leak turns into disaster sooner than later.Once a leak starts,the system starts to loose pressure,causing the coolant to start boiling,which in turn raises the pressure even more,which in turn is pushing more coolant thru the small leak and so on.
My first '99 528i had a pinhole in the expansion tank and i was checking it every day and everything was under control until it was not anymore.