Could be a variety of sources for such a leak after inner/outer primay R/R for seal replacementand "behind the derby cover" doesn't tell us much.
Leak could be trans or primary fluid; do you know which? Does it leak after a ride and stop or does it leak just sitting on the jiffy stand? Who did the work? What seals were replaced? Answer the questions, clean up the leak area and take/post a photo.
Inner primary had to come off to replace the "main transmission" seal; can't get to it by just removing the outer primary. Now, are you referring to the main shaft bearing seal or main shaft/fifth gear seal? If you are referring to the main shaft bearing seal, there are two other seals that would have to be replaced; the quad seal and the oil seal at the sprocket spacer. The sprocket spacer is usually replaced as wellCheck out the attached diagrams. The diagram with the gears shows the main shaft fifth gear seal; part #4. The diagram with the transmission case shows the main shaft bearing, quad seal and oil seal; part nos 11, 16 and 18. We need to know which parts were replaced to help.
If the main shaft bearing seal was replaced as well as the quad seal and the sprocket spacer not replaced, the leak could be there if the bike is high mileage. Or, it could be the tech just botched the job; it happens.