2003 GMC C1500 Losing coolant

When you run the defroster are there any greasy film building up on the windsheild?

First two things I think of when you have a smell but no source is the leak is minor but getting onto the exhaust manifold and burning off, thus the outside and inside smell. Second and less likely is a heater core. Usually when the odor is that strong the carpet will be wet on the passenger side floor.

I would still focus on an external leak and pressure test the cooling system, leaving the pressure on overnight to see if anything is exposed. Sometimes the leak is more obvious when the engine isn't hot and is not going to burn off any coolant.