Wait, so you are loosing coolant but it's not from any external leak that you can see? Since I don't know what year of car you drive (help us out here) I would think that the LIM gaskets need to be done, or possibly check the coolant elbows and waterpump for leaks. Often they will seep and the coolant will dry up before hitting the ground. And the LIM gaskets are known issues on the 3800's all the way into mid-05. Also, the UIM is known to crack on the N/A 3800's as well which will feed the motor coolant internally. Pull the dipstick and see if the oil is clear or if it looks like a milkshake.
