question/suggestion: open the engine coolant reservoir.
Turn the engine on, add some water to fill it above the fill line
so that it spils over the top of the reservoir, look at the
fluid. Have somoene step on the gas pedal while
the other person looks at the fluid. Are there air bubbles
coming out from the fluid? Try this several times with a cold
engine. If there are air bubbles coming out than it means
that the engine coolant is emerging from your cooling system
either in the engine or elswhere.
ALSO you might want to replace the thermostat-not the housing
just the thermostat. Is your heater working properly-do you
smell engine coolant in the cabin?

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Poland 300 miles E of Berlin Germany