transmission

What they are trying to say, is if there is a leak and you can't see any on the ground then it must be leaking into the radiator coolant where the cooling lines travel to and back to the tranny. So look into the radiator for oil. Oil floats on coolant (water). It might not show up in the coolant reservoir just yet. My Jeep Cherokee does that and I found it was leaking into the transfer case. (The level in the transfer case rises over time.) It's the same Dexron IV in both so I ignore it, I just keep the tranny topped up.