Brown fluid in radiator overflow, jes had tune up done?

Car jes got a mild tune up yesterday, spark plugs, wires, air filter, oil change/filter and pcv valve. Today i noticed brown fluid in the overflow. It wasnt milky or foamy. Car has 169xxx miles on it. Its a 94 buick regal with 3.8l engine. Ive never had a tune up done on it in the 3yrs ive had the car, and i dont know if there was any tune up work done by the previous owners.