even if nothing (pink, etc) is visible in coolant?

I updated title as it wasn't clear.

Amercedes Mechanic told wife that there is a leak but never showed her.
This is the same guy who ripped her previously so I don't really trust him.

I do not see anything and neither do techs in 2 radiator shops.
One wanted to check it on some machine for $150.
The other said, there is no leak and he does not see anything.

This is an 85 300D with the white plastic coolant expansion tank.

 

The only thing that I can think of is that there's a problem with your radiator that is allowing the two fluids to mix (hole, crack), but that doesn't explain how the transmission fluid is moving up the tube into the tank. How is engine temperature and coolant levels?

 

It might benefit replacing the tranny fluid as well just as aprecaution if coolanthappened to get in...

 

given what it would cost you top replace the radiator, flush the tranny and the cooling system, I would go for the $150 test, but the whole thing sounds a little far fetched to me, that transmission heat exchanger is made out of pretty rubust material
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Could there be a leak or crack in tranny cooler even if nothing (pink in the coolant etc) is visible?

 

Never heard of such. Is your fluid level on your transmission dip stick showing a lower level?

 

I am not sure if the trans level is lower as I don't remember where it was exactly. The level was not low, however.

Transmission fluid looks a nice translucent pink on the dipstick.
A few days after wife brought it back from the shop (from a different repair: vacuum pump), the car overheated and some smoke came from the engine.
I opened hood and found the large radiator top hose completely disconnected and no coolant at all (a little weird as it just came back from the shop).
I replaced with prestone (+ 1/2 tap water). There is no pink of any kind in the coolant. Temperature stays below 80.Coolant looksa very light green/yellow. The plastic tank is old and discolored. The level of coolant was an inch lower a few days after I added it.
I did not flush it.