2008 2.7 diesel: Coolant low came on and I topped up one litre. After some days and maybe 50 kilometers it had lost maybe 0.2 litre. There are no dripping undrerneath that I can see when parked but it is winter here so ice/grit/puddles. I pressure tested it to 1.2 Bar as the coolant cap says. It held pressure for some 15 minutes before I removed that. I checked all visible connections and no drips nor clear signs of previous drips. (Inspected with a LED lamp.)

The plastic valley hose was replaced by a dealer with a good reputation a few years ago. I can start the engine cold, and move the car, switch off, and there is no pressure at all in the coolant. Coolant is nice and clear. So not a blown head gasket (I haven't heard of a single case of blown head gasket in these diesels anyway).

What I am thinking is that either I have a hairline crack somewhere in the plastic parts in the cooling system, possibly one that gets worse when everything is nice and warm. Alternatively an O-ring failing also only showing when the system is warm. I am thinking of retesting next time it is properly warmed up.

Anything obvious I have missed?