Check your lower radiator hose from the radiator to the engine. There might be a small leak that is dripping coolant without you noticing. Also check the thermostat housing, which can crack and develop a small leak that can evaporate before you even notice drips, etc.
Be sure to check your oil frequently for any signs of contamination. If the oil is getting discolored (e.g. milky, lighter than normal, greenish or tan sheen, weird burnt smell), it could indicate a leaking head gasket or other internal engine leak that would need to be repaired immediately.
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