Excessive white exhaust smoke

I have the car listed above 4MATIC, mileage 152,000. Recently I've noticed light gray smoke coming from the exhaust when at idle. If I rev the engine, the smoke becomes thicker and more "white", and when driving it becomes much thicker. I've checked the coolant, including the overflow cap (looks normal, not milky). I checked the oil, looked and felt fine, no coolant smell. Also I removed the overflow cap started the engine and let it idle to normal operating temperature. There were no bubbles or anything else that would indicate a bad head gasket. The vehicle does not leak coolant anywhere that I can see nor does it burn or leak any oil. The car runs fine, the engine is smooth and responsive. I did not do a compression test, however I did remove the plugs and they looked fine (no indication of a coolant leak). At first I thought perhaps it was simply condensation in the exhaust system that was causing the excessive smoke. However, it doesn't go away after driving a few miles. One more thing, the Check Engine Light has not come on. Any ideas on this? Thanks for your help.