If you can`t see a leak then it must be using water internally, one thing to check is the coolant chamber linked to the egr valve, I think it is the stainless steel part just above the g/box bell housing bolted onto the head, to test remove it,block one coolant pipe and try pressurising the other and dip it in a pail of water and check for air bubbles,if any renew the part,good luck.
cdc, January 2015
