Outside of the brown tank issue, which makes me wonder if someone left the filler partially open in dusty conditions, clear fluid is not necessarily an indication that the fluid is good. There is a specific ratio of purified water to urea that the system requires. I don't remember the percentage, but it is a window of only a few percent. My understanding is the old fluid, though clear, may suffer from evaporative loss of one of the two components. I believe the water part. This causes the fluid to fall out of spec. If non-Mercedes systems are advanced enough to calculate and throw a code when this is out of spec, then it is likely that Mercedes systems can do the same. Perhaps it is a calculation between the injected amount and the difference between the pre and post catalysts NOX sensors, but I can only guess. Either way MB wants you to pay to have the fluid drained and replaced every two years. Not inspected, and retained/reused if clear.
On a related note, assuming all of the above is correct, if the container that you are using is sealed so as to prevent evaporative loss the "born on" date of the fluid should be less of an issue. This assumes the container has some sort of a vapor proof seal that has to be removed prior to use besides a screw on cap.
This is my understanding of the system after talking to a few diesel mechanics who work on larger rigs combined with using some basic logic. That same logic makes me question if keeping the tank topped up all the time vs. refilling when you get the "reminder" is the best strategy. I was not aware until this post about the corrosion issue of internal sensors, only spilled fluid. But it makes perfect sense that a dry (or partially wet) sensor would suffer from corrosion. Each time I have been reminded to fill up, I have added fluid within 30 minutes or 30 miles. I just happened to be on the highway each time, and felt compelled to easily address a light on my dash by stopping in to get fluid. Dash lights, and low fuel lights just make me nervous I guess.
So not I am thinking the best preventative idea is to keep everything topped up and wet, and to drain the tank each two years like MB says just to make sure the ratios remain within spec.
FWIW...some additional information for context.