2002 chevy imp. leak

I have 2002 3.4 liter six cyclinder automatic chevy impala with 45,000 miles. I noticed a greenish colored liquid leaking from the upper front corner drivers side engine compartment that left a small puddle maybe 6 inches across on 3 to 4 seperate occasions. I also had a "low coolant" dash light go on and has stayed on ever since then. The level of red dexcool is just fine in the radiator and over flow tank. I took it to a local mechanic and he pressure tested the radiator which tested ok. I thought the green liquid might be connected to the ac system but the ac is working fine according to the mechanic. We think the low coolant light is just the sensor. What is this green liquid and where is it coming from? I need to mention that the cooling system was flushed twice and contains only red dexcool. The radiator and the over-flow tank are completely full.