My 2013 CLS550 w/62K miles boils or overflows a little water out of the coolant tank occasionally when full. Sounds scary, but never any overheating even in 118 temps, no water in oil, nor oil in coolant so I am relatively sure this is not a head gasket issue. Replacing the reservoir lid didn't help.
BTW, it's done this since I bought it two years ago, although after the 2300-mile drive home, I've only driven about 1K miles. Car runs excellent, never a code. Any ideas?
Meanwhile, I've got an appt. to take it to my mechanic in a couple of weeks.

There's a notch on the coolant expansion tank neck that stops the cap from being tightened too much. If you torque the cap past that notch trying to make the cap tight it prevents the expansion tank from breathing through a small hole in the neck. The rubber seal on the inside of the cap acts as a plug to keep coolant in the tank and any excess pressure escapes out the hole in neck but the coolant remains in the tank. If the cap is put on until its really hand tight then you'll have coolant leaking around the cap. At least this is how I understand it works. Also during the pandemic, there were temp workers brought in by Mercedes to package parts and some were sold the wrong caps because they were packaged wrong which caused problems with the tanks neck. Make sure its the right cap and once its on, it should feel a little loose but if the wrong cap was on it, the tank neck may be damaged resulting in needing a new tank. You can see the notch at bottom of the necks threads, the cap also has a notch that shouldn't go past the necks notch.

Thanks - it did turn out to be the tank. When I took the car to the shop they pressure-tested the cooling system and found the tank was not sealing even with the new cap so they replaced it and all has been fine. What threw me was I was not finding puddles of water under the car after driving even during the summer yet the car losing water - I was just sure I had a head gasket issue

Was the coolant sitting on the belly cover below?

If you go out and turn the cap when the engine is cold, it should feel loose which is normal to my understanding.

The cap does feel somewhat loose. I'm not sure if the belly pan was catching some of the coolant. I'm guessing most of it was expelling while driving.

Maybe it was evaporating but I've been going through some coolant leaks lately and one thing Ive noticed is there'll be green dots everywhere under the hood from coolant flying around and evaporating everywhere it landed, it leaves a trace. That cap should feel loose so don't try to tighten it tight as once it goes past that little stop, its too tight and will damage the threads and the ability for the pressure release to function properly. It just happened to my 2013 S550, apparently they sold me the wrong cap and when the car was in for service, the tech screwed it on too tight and it basically destroyed the neck. I got it home and the next morning there was coolant on the ground below it and coolant was around the base of the cap and the cap was really tight. So when I opened it and closed it again I only tightened it until it wants to stop and left it at that and it stopped leaking after driving it. The correct cap will make like a click noise or just stop at the little notch on the neck and thats as far as you turn it. It will want to turn more if you try to get it tight but thats when people screw it up. Thought I'd mention it here so others know. My tank is being replaced now because of that. Its a technical tank with a little engineering to it.

That very well could have been what happened to mine. I bought a cheap knockoff cap from Amazon and tightened it as tight as I could get it.

They were not designed to be real tight. The caps are $40 at the dealer parts counter but you can get them on some dealer websites that allow online orders for $20. I live in Georgia but I buy parts on the dealer website in downtown Los Angeles and they are typically half the parts counter price. I asked them why and they told me they are trying to build online orders for the dealership so they offer discount pricing. Getting dealerships to tell you the part number to use on their site is another story because they found that when they did that people were using the part number to buy it from somebody else,