yeah my bad dislexiakicked in...and edited old post.

Raises boiling point when under pressure.

 

Happened on my S around 30,000 miles as well. Same symptoms. I replaced the coolant reservoir cap and the issue never occured again. Also, as for putting distilled water in the coolant reservoir, if the vehicle sits for an extended period of time it can cause issues so be wary of that. If MB finds any coolant issues in the future, they might deny warranty because of this.

 

It took very little to put it back over the full line, wont impact the mix of coolant and water in the system.

In fact I'm surprised it being that slightly below the fill line triggered the warning.