If there are no obvious leaks then its likely to be water loss from the cylinder head or radiator cap. Check if there is water vapour in the exhaust gas - hold a dry cloth onto the hot exhaust and force the exhaust through the cloth [hot engine]. Is the cloth wet?
Also take a look into the hot radiator [take care removing the cap] and whilst the engine is high revved, look for bubbles from the radiator. If there are bubbles then the cylinder head is leaking.

Keep your fingers crossed that it might be the radiator cap. Renew the seal in the cap or squash the cap's lugs so it fits very tight to the radiator. The lugs become slack with age/use and vapour escapes to be boiled off on the hot radiator - hence water loss and you dont know its happening.