Hi all, I am losing about 1 litre of coolant on a 50 mile journey driving at an average of 50-60mph. I have replaced the coolant filler cap with a genuine Fiat part. The head has been pressure tested and no loss there and no signs of water in the oil. i have put k seal into the system. I know that K seal doesn't seal pipes or water pump leaks but there is no visible sign of leaking pipes or any moisture around the water pump. No signs of leaking liquid beneath the radiator either. I am at a loss as to what is causing this. Anyone experienced similar or might have an idea where to look to next?
Many thanks

do you have an auxiliary rear cabin heater?

No just the conventional heater matrix in the front driver/passenger area

Put a dry board underneath when u leave it to see if any water drips also check pipes when hot