Barring any external leak, like the upper hose and recheck the bottom hose fitting - too tight and you will breake the plastic fitting, you will have to consider the reservior cap being bad, the connection at the bottom of the reservior being cracked, or any where else - like the heater core.

For engine coolant loss, remove all 5 spark plugs. Then with a small flashlight, shine it into the cylinders. If you see a 'clean' cylinder, that could be from coolant. A compression check will also confirm.

I would first concentrate on externals around the engine bay. Do that when the engine and coolant are hot. Use a clean paper towel, the blue/green will show up much easier. The cabin heater core can be easily seen after removing the kick panel.

Klaus
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There is no present time, just the past and future