I had exactly this problem when the pump needed a new diaphragm. However, there was a very small ‘weep’ from the pump so it was not too difficult to pin down, once I started seriously looking.
Is there a non return valve in your pump that is allowing pressurised water to escape back into the tank?
Do you have a calorifier that might have a small internal leak?
The amount of water should be visible if it is leaking somewhere external so it must be something ‘internal’.