It could be that you have a leaking heater core, which resides inside your car. Another give-away for a leaking core would be a sweet smell inside the car and/or a sticky residue on the inside of your windshield.
your heater core is leaking. Needs to be replaced
Leaking heater core
The heater core is leaking coolant.
the heater core, heater core hoses or heater control valve is leaking
If your heater core is LEAKING engine coolant so that your engine coolant level drops too low or the engine coolant is lost completely and causes your engine to overheat then your engine can be damaged beside the mess the leaking coolant makes inside your vehicle ( you might be able to just temporarily bypass the heater core so that no engine coolant is flowing through the heater core )
The heater core is leaking coolant.
Most likely it's a ruptured heater core, leaking coolant. But before you replace the heater core, check the connections outside the firewall. Sometimes a leaky connection will drip to the inside... but don't expect it; it doesn't happen often.
The Ford Taurus is famous for having leaking heater core. It is located on the passenger side inside the passenger compartment and inside the dashboard. It costs about $1000 to get a mechanic to fix it.
If it's leaking from INSIDE the firewall (away from the engine compartment) you probably have a leaking heater control valve or damaged heater core.
Yes it can. check the oil dip stick. if coolant is leaking inside the engine the oil will be "milky" in color.
Coolant reservoir, or directly in the radiator. If you are having to add much coolant, you need to find out why. Serious engine damage can occur if it is leaking inside the engine.
If coolant is leaking inside your cabin, you have a leak in the heater core. The only repair is to replace the heater core. Most auto parts stores sell them.