Hi,
I think I have a similar thing on my 2.2
I have had it since July, and in August did a complete flush of the coolant and filled it up to the correct level with the correct mix of antifreeze. Over the next couple of weeks kept an eye on it and topped it up as necessary as it was removing air bubbles (a very tiny amount!)
However, since August (about 3months) and about 3.5k miles, I have noticed the fluid is steadily going down, only by a very tiny tiny amount (I check all levels each time before I drive it), but enough to bring the level about 1cm below the correct cold level line (seam on the bottle). I topped it up yesterday and checked it again after another 400miles this weekend, but no level change.
The oil cap is clear, the coolant has not discoloured at all, so not going there.
I have noticed that when the vehicle is left idling once warm, it tends to produce a bit more white fumes out of the exhaust than I would expect from a warm engine, might be because it is running on LPG, not had a chance to try it warm on petrol yet.
Also, in August when I did the coolant and service, noticed that the coil pack & plug on cylinder no4 had a rusty brown coloured deposit all over it. Totally bone dry so I assumed it was old, and cleaned it all off. Not checked it since but will soon hopefully.
I know this might not have helped but seems like we are having the same thing? I wonder whether it is as a result of the car overheating after a run. I have noticed that if you park up after a run, temp at normal range, then go back out to the car about 10mins later, the temp needle will be over the 100C. The 2.2s do not have the electric recirculate pump like the v6s, and I wonder if it is worth fitting something?