I've had coolant loss issues over the last few weeks, but with no descernible cause. Eventually diagnosed as the HBV, but it only started noticeably throwing out coolant when I was fiddling with the temp controls.
Entwood very kindly posted the following which helped me considerably:
Non-consistent loss of water is, I believe, often the HBV as it responds to changes in demanded temperature in the cabin and the outside temperature... it therefore sometimes leaks, sometimes doesn't
What can help is to leave the car idling at normal temperature and make several large changes to demanded cabin temperature, leaving things to run between each change, and watching the HBV area like a hawk. It is then, sometimes, possible to see a leak as the HBV cycles to change the cabin temp.
Did this on my mates troublesome beast and sure enough, the HBV leaked when supplying HOT water (initial heating of cabin), but not once the cabin warmed up and the HBV backed off to maintain temp.