Elusive coolant leak

[Nudist]

I am having some difficulty in resolving a coolant leak in my M7 golf diesel. It all started three or four weeks ago when the coolant level indicator came on early one morning as I was about to leave home. I drove the car for a few minutes then the indicator symbol disappeared. I thought nothing more of it.
The indicator came on a second time a few days later as I was driving the car around. I checked the fluid level and noticed the coolant levelwas below the little metal sensors fingers inside the coolant reservoir.
To cut a long story short, I booked it in to my repairer and they immediately pressure tested the system for half an hour to an hour and the pressure held and they couldn’t locate any obvious leaks underneath the vehicle. I make an effort to keep my car very clean so it should’ve been very easy to spot any obvious leaks. Subsequently they have replaced the coolant in the car and I have made one trip Canberra and Sydney return. That was three weeks ago and I haven’t driven the car since.
However I did notice that when I was in Sydney the coolant level light came on as I was about to return to Canberra immediately stopped and unscrewed the reservoir cap. and the level was still good. No indicators light during the return trip and the temperature gauge was reading as normal.
I was planning to make another trip to Sydney on Saturday and went out to the car and inspect the level of the coolant and low and behold the level has dropped and I’ve added an extra litre of water to it to bring it back up to the correct level. So given all of the above information I’m a bit flummoxed as to what the problem is. I have checked the oil and there is no milky residue suggesting a cylinder head leak.
Has anybody in the group experienced something similar with their golf and if so how did they eventually locate and fix the problem?
The leak appears to be minor at this stage so I am planning to drive the car but make sure I carry some water with me to top up the Reservoir. I have booked the car in for a second time on Wednesday for the mechanic to have another look. They did make the comment that the golf does use a lot of plastic parts in the radiator system and he wouldn’t be surprised if one of these is starting to leak.