I've seen enough coolant enter the transmission for it to leak fluid from the overflow vent and not have a milkshake consistency. It was brownish. If you dumped a quart in all at once it probably would but a longer term slow leak wont.

You could always drop the pan and look. A trans fed antifreeze from the cooler will almost always have rust pock marks all over the pan and steel valve cover plates.

Any good MB shop will have glycol tests on hand. You could take them a small sample of fluid to test if in question.

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90 300TE 4-M
Turbo 103, T3/T04E 50 trim
T04B cover .60 AR
Stage 3 turbine .63 AR
A2W I/C, 40 LB/HR
MS2E, 60-2 Direct Coil Control
3" Exh, AEM W/B O2
Underdrive Alt. and P/S Pulleys,
Vented Rear Discs, .034 Booster.
3.07 diffs 1st Gear Start

90 300CE 104.980
Milled & ported head, 10.3:1 compression
197° intake cam w/20° advancer
Tuned CIS ECU
4° ignition advance
PCS TCM2000, built 722.6
600W networked suction fan
Sportline sway bars
V8 rear subframe, Quaife ATB 3.06 diff