On mine the Transmission Fluid is cooled by going into a transmissin cooler built into the Radiator.
If the internal cooler tubing develops a crack the Coolant and Transmission fluid can mix.

You might also want to check your Coolant; after it sits over night transmission fluid should float to the top of the Coolant.

If the above things turn out to be true you will need a new Radiator.

If it was raining out I guess if the dip stick tube was broke off near the base it might be remotely possible for water to get it. But, you would think if that was so there would also be a Transmission Fluid Leak their also.

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84 300D, 82 Volvo 244Gl Diesel