ABS

Could be almost anything from a bad wheel sensor to bad ABS controller. You need to scan the system with a scan tool that can read the ABS system and see what the codes show. MB cars are fussy about scan tools as well, most of the ones that claim to read ABS codes will only pull the generic codes and not the manufacturer specific codes used. For that you need a dealer level tool to get all the codes. That will give you a place to start from.

There should be continuity though the sensor full time as it is just a coil of wire inside. Sounds like a sensor failure. The system uses information from the sensors to make the speedometer and the other items operate. On most cars the failure would trigger the ABS light, on the Mercedes they tend to make any failures more noticeable.
I would double check the continuity in the sensor and if it fails replace it. Then have the codes reset.

Check the ring that the sensor reads as well. They can crack and cause issues as well.

Cold could cause it if the connection is poor to begin with. But I suspect it will be back. The front sensors are of the same type so they should also read continuity. There are also brake wear sensors on there, those should not have continuity, maybe those are the ones you tested? Either way if the light went off the error went away. Keep us informed please.
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Internal wire breakage and internal connector failures are very common, especially with ABS/speed sensors and door/trunk wiring. It's one of the reasons I have about three fox/hound wire break detectors in the toolbox.

Codes -
There are multiple types of code setting criteria, some like the ABS and engine sensors are under constant monitoring by the control systems. On those 99% of the time the light will go off and the code will become a history code as soon as the system powers on and the control system detects that the fault is gone. So if you have a bad TPS sensor that sets a code, you replace it, and that was the problem, the light will go off.

Others are not monitored that close, they may run on a time cycle like every engine start and take 2 fails within 2 starts to set. Those will usually clear themselves after the system runs through 3 or 4 starts without that fault.

Then you have tests that are only run under very specific conditions. EVAP tests are usually that way. They may only run when the ambient air temperature, engine air intake temperature, coolant temperature Fuel level and engine rpm are all in set windows.

For those tests to run there cannot be a fault in any of the systems that are used to monitor the test. So say you have a faulty fuel level sensor and don't repair it. It is very likely that the EVAP tests will never run because the computer doesn't know the fuel level. Say you replace the sensor, now it can run the test, but you also have a bad fuel cap and purge solenoid that now show up as EVAP leaks and set codes! These are the "I took it to the dealer/shop and they replaced my fuel pump but the light just came back on, did I get screwed?" Type items.

Most codes will actually clear themselves IF the fault that set them was repaired and the test runs and shows the repair was correct. We use the scan tool to clear them simply because of speed, If the test takes a week to run but we can clear the code and initiate the test in the bay, we can determine if the repair was correct and functional.

There are also codes that will show up as permanent codes that cannot be cleared even with a scan tool. Those are vehicle and system specific.

Scan tool - Mercedes has a multiple tier system with the "common OBDII codes" then manufacturer specific codes that a good after market tool can read and then some that are on the various CAN bus and fiber optic lines that only a true factory tool can read.

For a scan tool that will clear codes and be useful on multiple brands, it depends a LOT on what you want to do and how little you want to spend. Give me a number as a top amount and I'll create a list. Generally the $100-200.00 units will read codes and clear them and may give you some live data but they are somewhat vehicle specific, some will work great with domestics but not imports, others are great with import coverage and lousy with domestics. The next tier up gets better and so on. For bidirectional control and testing you will be looking at something that starts around $800.00 new.

Any other questions just ask.