Clio losing water

[Willo40]

Hi Guys, a mate has on 05 plate DCI Clio that is losing water, but I can't find a leak anywhere, he has even had it into the local garage, who can't find it leak, its just a little bit of water either, I am talking a couple of litres in a week or so, no sign of steam out the exhaust, the filler cap isn't full of sludge, the oil looks the right colour, there was some oil in the water bottle, and the car got hot once, but that's it, any idea's where it could be coming from, and where its going?

Thanks....