X5 4.8is Losing Coolant - but where?

[X5monsterBMW]

Hi all - my X5 4.8is V8 E53 (Dec 2004 110K miles) has started to lose coolant... but where from - that is the question - I just don't know!

Makes a change not to be wrestling with air suspension & yaw sensor problems... car has been trouble free for 6 months!
Started new thread as not much on here relating?

In the 4.5 years I have had the car it has never really used any coolant to speak of - only once when reservoir leaked at the bottom and was replaced.

6 weeks ago 'coolant level low' sign came on dash info... I re-filled with coolant and anti-freeze as winter. To my surprise it took a lot to fill... since then I have been checking & topping-up every week. Not gone low enough for warning info to appear again.

There is no water in the oil - clean as a whistle... there is no oil in the water - also v. clean!
The temp guage is steady as a rock half way up - no change there. Vehicle still drives like a 'bat out of hell' & no misfiring either... no warnings at all in fact!

Driving at night I have looked for signs of steam from exhaust in rear view mirror when other cars have been behind with headlights - no sign of anything weird... Slight steam/smoke from exhausts when idling outside 1st thing in morn - however it has been cold here and I think the car has always done that????? Just not sure as never really taken notice. Someone said it could be a low level head gasket failure - can't see anyone on here who has had that - just a few with milky/foamy oil deposits- but not me!

Every week uses about 33cl per week (I know this as I use an old beer bottle) which in imperial is approx 0.6 pint. I normally do a couple of hundred miles a week - last week I did 400miles or so but the usage is v slow and steady.

I have had the car up on ramps now at my local BMW independent - the mechanic & I looked under & above every area of the engine - we can see no signs of a coolant leak...

Not really sure where to go from here - any ideas folks?
It is really bugging me now!

best
malc