E30 Zone

Backstory:
1988 320i Auto 196,000 km original engine

Car is running well after having the coolant system refresh...
- no overheating
- temp gauge in the middle or slightly below at operating temp
- new hoses all round, throttle body gasket, expansion tank & sensor, new cap, water pump replaced last year
- new heater core, o rings & heater valve
- BMW blue coolant all bled properly

But after a few hours driving & the car cools down, I notice the coolant level in the reservoir has dropped 1 -2 cm.

Inspection & checks so far:
Appears no coolant contamination in oil (i.e. no 'mayo' under the oil cap or 'coffee' in the dipstick)
No oil in coolant
ATF is correct red colour
Appears no external leaks (no coolant smell, no puddles or dried coolant marks)
No white smoke apart from normal vapour emission after start-up

Rather than guessing, took it to a specialist down the road.
Pressure tested the cooling system and holds pressure when cold.
Asked for a coolant combustion leak test and they said it shows traces of combustion gases in the coolant.

Great...
Sounds like a head gasket hopefully not a cracked head.

Any comments or thoughts from @Brianmoooore or the brains trust here?

Risks in driving it?

The coolant level usually does drop a bit immediately or in the period following a coolant change - just keep an eye on it and top up if necessary.

You could always add a sealant additive such as Bars Leaks, K-seal or similar if it continues after say, a month.

As above they do normally drop a bit, I assume it is filled to the max mark and not brimmed to the top?

One other thing to keep an eye on is the upper corners of the radiator, mine went there on my old 325i and as it was mostly steam leaking out there was very little coolant trace.

It is worth removing the rocker cover while it is hot and still pressure in the system to check for beading where the cylinder head can go porous , look under the inlet side rocker shaft between cylinders four & six.
Look at the YouTube video fourteen seconds in to see where.

https://www.youtube.com/watch?v=jRe9dJB ... NEERINGLTD

As long as there is some coolant in the reservoir bottle, the engine won't overheat.
The official level for the coolant is the seam of the bottle, but on every E30 I've had dealings with, it settles 10mm or so below the seam, so keep an eye on it, and as long as it doesn't go much lower than this, things are probably fine*.
Check that the small hose from the top of the radiator to the bottle is clear.

* Having said that, connecting up a coolant system test kit to any car and pressurising it up often shows up minor leaks on cars you thought were sound.