Problem most likely is the coolant level sensor located in the coolant reservoir. Remove the sensor's power plug and closely inspect the sensor's exterior for evidence of corrosion and/or moisture, indicating a coolant leak; a leaking sensor (common)can short internally, thereby closing the circuit and activating the dash warning light.
If the sensor is clean externally, the coolant level above minimum but the warning light still on, disconnect the coolant level sensor's plug. If the warning light extinguishes, then the fault lies with the sensor(the float is most likely coolant- logged). Replace the sensor and seal (approx. $25 list) or disable the warning system by leaving the plug off.