John,
You know more about these things than most of us here! But I can understand why you are puzzled by the high level.

As has been said, brown colour doesn't point toward water/glycol being in fluid. More likely overheating (from those 1/4 miles?? ) Maybe some frothing causing higher level? If coolant leakage suspected, I guess you could remove the trans cooler hoses and pressure test the cooler for leaks?

I have an E320 with the same 722.6 transmission. Originally they were sealed for life! But now there is a special dipstick. Home made ones will work, but it's not just a matter of pushing a wire down the diptube. The factory dipstick has a stop on so it doesn't go all the way to the bottom.

Temperature is also very important. I use a piece of plastic tubing as a dipstick and pass a thermocouple (DVM accessory) down the middle to measure the temperature. This diagram from this BW thread gives some idea of the affect of temperature. Level should be measured with ATF at 80C. I think difference between min and max when hot is only about 0.2L, so that would not explain your 1L (How did you determine that volume?)

__________________
Graham
85 300D,72 350SL, 98 E320, Outback 2.5