98 GMC Sierra radiator low on fluid

[pitterpat]

My GMC radiator is always low on rad fluid. The radiator overflow tank is always low or empty. Lately I have been smelling that antifreeze smell. Funny thing is I never see antifreeze dripping or leaking. 5. 7 engine.
Yes, I have had to add antifreeze to the overflow tank recently, have added a whole gallon in the last 2 weeks. What is causing this?

 

[SCOTTYINWV]

Take it to a shop and have the system pressure tested. If you see no wet spots or residue it's going to be the only way to find it. Have you checked the oil level?