Thanks for the pointers guys.
There are some traces of liquid on top of the EGR Cooler see pic - might be coolant residue - could it be coming from the black plastic attachment? What's the fix - new o-ring or replace the part?
The expansion tank is on the job list for tomorrow morning
It's along the lines of what I was thinking of in that the water loss happens during the cooling down phase...
Mistercamp, do you have any more info on where to look in the front bumper area - I dont recall seeing any hoses there?