SL55 2003 coolant light. Leak? where? What?

[lplplp]

Coolant light has been coming on when coolant at normal level. Been over filling marginally & light goes out. Excess fluid goes after 30 miles, level at normal & light on again. Haven’t dared running it longer to see if fluid goes beyond normal. How risky would this be? Running a little hotter than usual but still between 85 & 95.
Brilliant local, but non merc garage can see no obvious leaks after extensive look & tests. Pressure test also not adverse. There is some yellow scum under oil cap.

Any ideas what the worst, best this could be?

 

[Jobsworth]

If I were you I’d remove one of the intake plenums (aluminium bit, ///AMG embossed on the top) and look for dried coolant in the ports. Chargecooler leakage is not common, but I have seen it a couple of times before.

 

[Major774]

Yellow scum under the oil cap is normal for multiple short journeys.

Did the pressure test hold pressure?

 

[markjay]

So there's no actual coolant loss then?

In this case I would suspect the coolant level sensor.

 

[lplplp]

Yellow scum under the oil cap is normal for multiple short journeys.

Did the pressure test hold pressure?

Yep - apparently pressure was fine in the test & results suggested didnt react to get rid of any coolant that may have built up In engine. Was done by an excellent but not merc specialist local garage though so they mystified by result & cause of leak

Yellow scum under the oil cap is normal for multiple short journeys.

Did the pressure test hold pressure?

 

[lplplp]

If coolant is leaking internally somewhere, albeit slowly, how many miles or how long could you do to see if it dips below the standard fill level once the light comes on? Would you get any other sign of potential engine damage before you actually do any if the coolant is going?