If the reservoir was empty, you will likely have to keep checking it as air burps out of the system. I would guess you have a leak somewhere, but the code may have been from low coolant. I've not had any coolant issues on mmy Smart so I can't direct you to where the thermostat is, but I have had these issues with other cars and it should be pretty much the same. Problems I've encountered in order of frequency are thermostat stuck (open or closed), leaky/ruptured hoses, radiator fan died, failed water pump.
Have you cleared the code and had it return?
Is the coolant level stable or has it dropped again?
Does the fan behind the radiator come on?
Have you cleared the code and had it return?
Is the coolant level stable or has it dropped again?
Does the fan behind the radiator come on?