water leak

[mark740_5]

hi
i have a 51 plate passat sport tdi 130,and i do about 600 miles a week in it,lately it has been using/losing water,there are a few signs of water leaking on the drive,but i cant find a leak i have left it runnung on the drive trying to get it hot enough but there is no water leaking(that i can see) i have changed the expansion bottle cap and there is a slight chance its leaking from the bottle because its slightly wet ,i have had it pressure tested and the pressure does drop but again no visible water loss,also had 2 checks to see if head gasket failure,the CO2 machine in the expansion tank reading hydro carbons they are no higher than regular air around us, anymore ideas cos i am proper fed up at the mo
cheers

[passat>sport]

Water-pump (remove the top cam cover to see) o-rings on the radiator pipes, the little pipe that runs under the tandem pump and into the head can rub thin and leak. others of course but I've come across these myself.