If you have the 3.4 liter V-6 engine, it is quite common for the intake manifold gaskets to develop leaks; particularly if the engine coolant has never been changed.
If yours has the 3.8 liter V-6 option, on those it is common for the plastic intake manifold itself to become distorted and begin to leak coolant, especially when the weather starts to turn colder this time of year.
The way to be sure where the coolant is leaking from is to have a cooling system pressure test performed. For this test, a special tool is used to pump compressed air into the cooling system to pressurize it; this makes even small leaks very apparent. This test is very quick and inexpensive at almost any repair garage, or you can purchase the pressure test tool to check it yourself for around $150.
Without knowing exactly where the coolant leak is occurring, it could be many things, including a head gasket leak or a radiator leak; the symptoms will be very similar. A cooling system pressure test will show any radiator leaks.
If you have a head gasket leak that is allowing pressurized exhaust gas from inside of a cylinder to seep into a coolant passage inside the engine it can also cause symptoms like you are describing; including the full coolant expansion tank. If your mechanic cannot identify any external coolant leaks with a pressure test, the next thing he will likely want to do is a chemical test for exhaust gas in the cooling system and/ or a cylinder leakdown test to check for head gasket sealing problems.
In your original question you stated that you can see visible coolant (but are not sure what side of the engine it is coming from), so it is more likely that you have an external coolant leak causing the problem.
Your mechanic should be able to identify the cause of the problem fairly quickly and easily using one or more of the standard tests I have mentioned; there should be no guessing or replacing parts at random required.