Had the engine rebuilt and HG changed in February. I didn't check the coolant level in 2 weeks after receiving the car. I somehow refused to open the bonnet after all I'd been through. When I eventually did, in a freezing morning (0C) the level was midway between the two spheres (min and max). No "hiss" from the cap on opening and just a clear rubbery smell.

Checking again today, 4 weeks after last check, the level was exactly at "min" at an ambient temperature of 12C. A very brief and weak hissing on opening the cap, perhaps a hint of exhaust smell or at least not just rubber? Distance covered between checks was about 3K km.

Most of the cooling system is newish or new. So we can rule out the usual suspects for leaks.

Now, how much of a coolant loss is to be expected over time / distance covered? I assume it being a closed system the answer should be none?

After major work like that, you can expect a small top up. Keep an eye on it and check for obvious leaks. 2010 3.6 TDV8

Thanks, that was my thinking when I saw the level half way, 2 weeks after the job. I assumed they had filled it to the cold mark and it had subsequently settled to a certain level; fully expected. Now it seems to have dropped further. As there are no obvious leaks and the weather is warmer in comparison, I'm beginning to worry.