I have a 2009 BMW 328 with 72k miles. I opened my coolant res. to see the color of my coolant since it was flushed 3 years ago at 64k miles (I am sitting at 72k miles right now) and noticed brown looking liquid. When I push the float down and look directly below the float, the liquid is blue. When I look at the back end of the res. there's that brownish-colored liquid. The Oil filter housing gasket was done around 67k miles. No oil dripping or smell from anywhere. Maybe the coolant is just dirty?