hehe, i guess the alternator worked out . bmw's have plastic radiators, plastic expansion tanks, plastic air bleeder screws, plastic thermostat housings, it's not uncommon for any one of those to leak, and you can't rule out the water pump, a hose, or even the clamps for that matter either.

basically the anti-freeze circulates throughout the engine, heater core, and radiator, if you get low on coolant there's not enough fluid there circulating therefore your car will overheat because it's leaking out. if you continue to drive a car that's overheating you're going to blow the head gasket or cause other very expensive repairs so you need to address the issue before you create a more expensive problem.

basically you'd pressurize the cooling system according to the pressure specification on the radiator/expansion tank cap and then look to see where the coolant is leaking out of, then make the repair.

just think if your WHOOOLE problem was a 99 cent clamp that just needed to get tightened back down or replaced and because you continued to let the car overheat you're forced to spend $1500-3000 to dissassemble the motor and make additional repairs because you didn't just fix the problem....that's just poor judgement.