Check your coolant reservoir! I discovered mine had degraded over the years and the top black plastic was literally snapping off of the reservoir (in the first photo you can see the broken connection plus, in the foreground, the piece of plastic that snapped off while lifting the reservoir out of the engine compartment). The small hard line connection fitting on mine was broken to the point the hard line was being held in only by the pressure of the o-ring; if that hard line blows out there goes your coolant. Additionally, the large o-ring on the screw top cap was hardened with a potential of allowing the pressure to bleed off resulting in a lower boiling point.
I found it easiest to gently clamp off the lower hose with a pair of long-nose vise grips (yeah, I know I should buy the proper hose clamp tool...), leave the screw cap on, and used a foam ear plug to close up the hole where the hard line attaches. Unplug the coolant level sensor connector and move it to the side. Lift and tilt the reservoir back to access and release the spring clamp on the bottom fitting. As you remove the lower hose, continue tilting the reservoir back to minimize spillage; I lost about a bit less than a pint doing it this way. Empty coolant into clean container, transfer the coolant level sensor from old to new reservoir, and install reservoir in car. Refill the reservoir (your choice of old coolant if it is not that old or new coolant). FWIW, the reservoir holds just a bit under two quarts (~ two liters) so ensure your container is large enough.

To keep original look, you will need one or both of the following labels. Cost is ~$11 each from the dealer. First one is standard warning lable, second label is for the AMG vehicles.
-High pressure label A 210-584-07-21
-AMG label A 000-584-26-11

 

Good tip! Glad you were able to see this before before getting stuck somewhere. What year and mileage do you have?

 

2005 with 194,000 miles.