Another common problem is the front water pump seal will weap coolant. It's hard to find the leak at first because the belly pans do a good job of covering them up and makes hard to trace. Sometimes the Houdini coolant leaks can be some what challenging to diagnose.

 

A few weeks after I had the coolant tubes replaced in my 06 CS It came up low on coolant. Thinking the worst I took it back to my mechanic --he said he had a hard time getting all the air out after putting the new coolant in and suspected it was just more air working its way out. He topped up the coolant and I haven't had any problem since.