It looks like I may a small coolant leak. The warning light has come on a couple of times over the past 2 months and have had to add about 4-5 ounces of coolant each time. I can't see any leaks, the oil looks fine and no white exhaust. At first I thought it was the overflow tank, as the cap didn't seem to screw on tight. I put some teflon tape around the threads of the overflow tank and the cap seems to be more secure now. My usual Mercedes repair place can't get me in for a couple of weeks, anyplace I should look at on the car?

 

Check the coolant lines to the turbos, they leak. There are three of them made from plastic. Lots of youtube videos showing their replacement. Look up m278 turbo coolant lines replacement. The ones made from rubber typically dont need replacement.

 

Normally you can smell it if it's an external leak. If it leaks from the cap then you should see evidence of it running down the tank, in the form of a white crusty powder.
There's several places it can leak, including the turbo lines ibbij mentioned, so you'd just need to look for water or that powder. The rubber lines ibbij mentioned are on the right turbo. For some reason the ones on the left turbo are partly plastic, because why do it right when all you need to do is make past warranty? Those plastic lines are not easy to see with all the crap in the way, but do your best. The attached pix I stole from the net show where they are. And fyi those lines are not identical through the years so they may look a little different or be positioned a bit different, but they are there.
Another option to spot leaks, like from those lines, is to remove the lower eng cover, let it idle at full temp and look for drips from underneath. Otherwise that cover catches small leaks that can evap before they hit the ground.

My main concern would be a gasket deeper inside the engine between the block and timing cover that is known to leak water into in the crankcase. Time and heat are what cause it, since it's a typical rubber gasket, and your eng is old...
I'd also monitor your water temp, because maybe all is well but your fan died. I thought mine died once because one day in summer it would get hot-ish when I was stopped at a light. Ok, fan died, gotta troubleshoot that when I get home. I turned the fan by hand and it moved but very tight and crunchy sounding, seems like a classic bearing fail. But it didn't exactly feel right and I kept playing with it, ultimately I noticed a seed pod wedged between the fan and shroud. A Rat had stuffed the pod in there... I pulled the seed pod out and problem solved. Point being that a fan fail, for whatever reason, could cause your issue too. If it had been cooler out, like now, and I didn't look at the temp, I suppose I could go many months without a fan and not notice. Well, maybe some water goes missing now and then. So preventing the cap from venting with tape could over pressure and break things. Or could force coolant to into the engine pas that gasket when it otherwise wouldn't, or if it is then you made it worse. If the cap is bad then just buy a new one. I bought one a year ago and it was $11. Meanwhile I'd make sure your fan turns on, and monitor the temp with that tool of yours. This would be a great time to use the cheapo tool I suggested to record it the entire time. Can't trust the gauge on the dash, btw.

Back to that rubber gasket deep in the eng that can leak; I don't know if it's true, but rumor is the oem coolant will damage the bearings. I switched to Propylene Glycol that apparently doesn't hurt anything. I also lowered the eng temp ~30F which will both extend the rubber gaskets life and greatly reduce the pressure trying to push past it.

 

I replaced the cap a few years ago. At first I thought it might be that, as there was white residue around the cap on the tank. I cleaned it up and sealed the threads better with a bit of teflon tape. I haven't had any white residue since. I have been watching the temp and it seems to be OK. I tried looking down toward the water pump, but couldn't see much. I really couldn't do much, as I live in a condo. Anytime I raise the car hood in the parking lot the condo police get itchy.

 

If the cap is new-ish then I'd be more worried about the eng actually getting hot enough to vent, and worse, that tape blocking it from venting.
I don't know what psi they normally vent at, but I've never hit it, so if you do I'd wonder why.
The threads themselves don't seal the cap, but if you put enough teflon tape on it I suppose it's possible to block it from venting? That's what I was worried about in your case. I'd just lose the tape and make sure the lip where the O-ring in the cap seals is clean and free of damage. It'll make sense when look at the cap.
Also note the tanks tend to crack and leak, which I suppose could crack near the cap and give the impression it was from there? So I guess I'd look the area over, especially where you saw the white stuff. My tank has a crack but it stopped leaking on it's own. Your tank is much older so I'd kinda wonder if it doesn't have a crack.

Since you can't work on it, and I assume have to pay people to do it for you, then I suppose you have a dilemma? My dilemma is it's against my religion to pay people to do stuff like that. For you I guess it depends on how much $ you're willing to spend? If it were me and I couldn't work on it, I'd just let it leak. Like a co-worker said he got a quote from the dealer to fix an oil leak. The leak was maybe a drop a month or something silly, and they wanted thousands to fix it. We both laughed, but also wondered how many people would've paid it? Or that oil leak in the wiring harness our engines face, which apparently costs ~10K to fix? Hard pass for me but apparently some people pay it.

Meanwhile I'd keep an eye out for coolant on the ground, and especially for the smell of it. If you smell it while driving I'd be instantly super wary because it could be a turbo coolant line snapped off and you have moments before you overheat. That smell always makes me super nervous but it's almost always coming from some other car. Luckily I can check the temp data at any moment and shut down long before it hurts anything. I would not feel comfortable at all using the oem dash gauge, but at least you can use your tool to see how far off the dash is and get a feel for where is dangerous. My dash gauge doesn't move much and I'm not sure it would read high if the eng was on fire. It seems to me that all car mfg's made an agreement to never allow an accurate temp gauge.

 

If I still owned a house with a garage, tools and an extra car I would have most certainly attempted to find and repair the leak. Even then it probably would have taken me five times as long as it should have. The leak started before I put teflon on the threads, so I don't think that is the issue. That being said, I think I will removed the teflon tape. I live by myself in a city where I don't really know anyone, so my car is important. Other than Uber, I don't have a way to get around. That is probably the biggest reason to have some else fix it - they can do it correctly and much quicker than I ever could. That being said, I have a couple of places I could have look at it. One is a German car repair place I have used before and they are great, but use OEM parts only and are more expensive. The other is a normal high-end repair place that I have used for select items before. They will use aftermarket parts, if available, and are much less expensive than the other place.

Update: I was able to grab a couple of pics around the water pump. It looks like at least two turbo coolant lines may be leaking.

 

There are two O-rings behind those black plastic parts (small attached pix). And the part with the O-rings can break off, basically letting the water bypass the O-rings. If I had to guess, I'd lean toward that being your issue based on your pix, and assuming that's where most of the white gunk is.
There are O-rings on the Thermostat too, one main one (in pix) and two little ones but I seriously doubt they ever leak. I'd assume if the Stat leaked then it would have gunk on its bottom edge but it looks ok in your pix.
If the water pump leaks the water comes out the bottom, behind the pulley. I suppose it's possible it could leak from there and get slung over to where the gunk is in your pix, but it would also make a big mess at the bottom of the pump on down. Then there's the water pump gasket behind it, but it's been a long time since I've seen one of those leak.
The big pix is the water pump with the turbo lines, the two problem ones and the other with the QR code label is the oddball you need to replace too. The right turbo lines and thermostat are not in the pix. Not sure if your lines are the same as the pix, but they should be more or less the same. They changed the design a little over time, yet never considered making the plastic parts out of metal.

My thoughts on parts:
Not sure I'd want to buy an OEM anything, especially the water pump. An OE pump should be ~$200. And if you do the pump there's no point in not doing the Stat and turbo lines, plus the odds are good the lines and stat will break doing the pump. Stat cost ~$80 and the left turbo hoses ~$40ea, and you likely need three because there's that weird one in the middle.
If you only do the lines then I think you gotta pull the stat, so again it may break. It just depends on how old it is because they're plastic and apparently built to fail over time, like the lines.
If you're pulley are original you might consider doing those pulleys if they're original. OE should be ~ $120 for the set. And of course the belt which is $10-20.

The turbo lines are a bit of a pita because of where they attach to the turbo. If you get the right position you can see the bolt holding them on the turbo. I can tell you I have no interest in replacing those. When my time comes I plan on cutting the bad plastic lines off the Alum tubes and using rubber hose.

My usual go-to for annoying small leaks I don't want to deal with is Alumaseal. That stuff works on pretty much everything except the water pump seal. It'll probably work if your turbo line broke inside as described above, but if that part broke then how long before it breaks the rest of the way, or one of the others break off and dumps your water out in a minute. Alumaseal is simply little particles of Aluminum that clog up the leak point, so the hole needs to be small. It comes in a little plastic tube and I've never used more than half the product on anything. It's a must have for the trunk imo, in case you get a pinhole leak in the middle of nowhere.

 

I used Alumaseal powder years ago, but it appears to be discontinued. There is a liquid Alumaseal, but does not appear to have actual aluminum in it. I may try a small amount of K Seal.

 

I've never used anything except Alumaseal, with the exception of waaay back in the day they offered the same basic thing but Copper powder. If I recall, it was Copperseal. I preferred that stuff but, but like Cadmium Sulfate to fix lead acid batteries they stopped selling it. I think it boils down to profits. Some bean counter says; We're selling these Copperseal dealies but copper prices are going up, so discontinue and push the Alunaseal dealies for $1.50 a tube. Then they realize, all these other sealer companies are charging $10 for a bottle of BS, so maybe we should sell $10 bottles of BS too. Imo...
Maybe I'll just buy a pound of Alum powder from some chemical company online.

Well, if you try some sealer, let us know what, and how it worked.