Leaking a lot of coolant

I have a leak somewhere because I have to add coolant every other day. Not a lot, but always seem to have to top it off. It has been that way for a few years. Husband replaced the radiator and water pump when it started leaking but the leak was still there. This past two weeks the temperature has been in the twenties. I drove to work and and I did not have any heat, it was blowing great but it was cold. I looked at the temperature gauge and it was almost at the hot. It usually never comes the cold so I did not think the gauge even worked. Husband checked the coolant and had to add a Gallon. We left the car sitting for three hours and when we checked the coolant again it was gone but we cannot see where it is leaking. Checked the oil to see if there is water in it and there is not. He started the car and added more coolant, we could see some steam coming up right in front of the firewall on the drivers side. It looked like coolant was dripping off something and landing on the catalytic converter. Would there be a freeze plug back there? Where could the coolant be leaking from so close to the firewall? Please help this is my car and he said if it is the head gasket leaking he is driving it to the scrap yard.