Normal to COLD?
You would only lose coolant if the engine was overheating - or, as you say - a leak.
It's probably a faulty temperature sending unit - these are common failures. You actually have nothing to worry about. The unit sends a signal from the engine to your temperature gauge in the car. Even if the unit is faulty the engine is still operating at the correct temperature - it's the signal to the temperature gauge that is wrong.
Your temperature sensor will look something like in the image below:
It's inexpensive and quick to rectify. A new sender unit can be fitted within minutes. Give a mobile mechanic a call ..
I read your question about the car heater.. No, the sensor wouldn't cause the heater to blow cold air. That fault is either air in the cooling system/heater system, or a blocked heater matrix.
Remove your radiator cap/coolant bottle cap. Open the heater controls to HOT and set on maximum. Start the engine and let it tick over - see if it forces any air out of the system when the heater should begin blowing hot.
If the heater still blows cold it may have a blocked matrix. Try a bottle of radiator/coolant system flush to see if that dislodges the blockage. The instructions will be on the bottle of coolant flush
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