My daughter has a 2008 Impala 3.5 and the coolant keeps disappearing and a pressure test showed no leaks anywhere. It takes about a week for all the coolant to disappear. No smoke from the tailpipe and no obvious mixing of water and oil when the cap is taken off. What else could it be?

Water pump seal.

Slow leak might leak so little that you have to look for traces under the water pump bearing.

I'll check that again.... when I looked at it last weekend I looked around the waterpump and saw nothing.

Did you pressure test when hot?