funny that you mention that because i just bought a 2000 328i about 2 weeks ago from a Certified Pre-Owned BMW dealership and my coolant light has come on quite a bit.
make sure that there is nothing leaking from underneathe the car. are there any funny smells or anything? cause luck has that my car was leaking today and i had it towed over to the dealership.
strangely enough i was just told that it was the water pump and thermostat housing that was leaking. and under my 50/50 warranty ill be paying a good $400 just to get it all replaced. as my own mistake i authorized them to fix it before researching the parts and they were both selling online for a total of only $110.
just thought id give some input on my problem with the DAMN COOLANT LIGHT!