I'm really puzzled and any advice would be good

My TD5 auto has been losing a bit of water lateley, not much, having to top her up every 3rd or 4th day doing +/- 100km per day. Not being happy with this and a trip to the Kruger Park coming up, I had a pressure test done which showed no leaks in the plumbing. A co2 test also showed no head gasket leaks. The garage told me the only thing they could think of was that the cap had become weak and the water was leaking through the expansion pipe. Ok, I so I put a new expansion cap on her. Yesterday, I drove 176km. When I checked the water levels, I had to put nearly a litre of water in her. Now as I'm driving back, there's is a continuos gurgling sound in the heater matrix, so I stop to make sure the water levels are still ok. There was no hiss or pressure release when I undid the cap (temp gauge up to normal) but the water level was ok. I put the old cap back on and drove home(176km). Again checked her when still a little warm and there was pressure in the system as per normal and the water level was ok:doh:.
I'm stumped now as to where the water is going. There is no obvious leaks, the oil is not contaminated...........please help

Had the same problem and much the same diagnosis, however turned out another garage found a very small hole in the radiator.
Radiator replaced - no more annoying top ups.

Thanks for the info, I'll double check the rad. It was a new rad in Dec last year so I'll be ****ed if thats gone again!!