It's obviously going somehere inside the collant network or outside.
Not heard of head gasket failure on one of these. I have had leaks before from header tank which I replaced as well as a failed bleed screw to same.
I suggest you first check oil dipstick just to make sure it's correct colour & not emulsified from cololant mixing with same................hopefully all will be well in which case......
1) Top up with correct coolant/water mix.
2) Carefully check hoses for any white stain evidence of leaks from same &/or connections.
4. Check when engine running for any dripping from area water pump, (do mind the belt/moving bits). Look for any obvious leaks elsewhere including radiator though you wont see much & then may need torch & keen eye.
5. If unable to find leak I would bleed the coolant by running engine with heater on max heat heat set to demist. I do this initially with header tank lid undone/removed, then do same up again. With engine running slowly release bleed screw to release any air. I then let engine cool before checking level again. Not the most technical or correct explanation but works for me.
6. Also clean engine bay as best able near any coolant pipes, connections. Maybe dab a little talc near suspect connections to help later detective work but if coolant mix good you should see stains post leaking.
7. Periodically run engine to operating temp for some time then visually check for leaks again. Look for moisture at exhaust exit maybe but do consider that if car has been sat cold for a while then condensation/moisture can build up.
Usual suspects would be header tank, bleed valve, but consider water pump, pipes & connections assuming major failure via gasket ruled out.
Perhaps someone with more info will assist further........You are not alone as many have experienced mysterious coolant leak on M62 engined cars.
Good Luck
