THe Active tank in a 35d is not under the hood. It is behind the fake air intake in the bumper on the right hand side (i.e. in the wheel well between the fender and the wheel well cover).

The area near the DEF filler fittings on our 35d has a very small amount of white crusty residue. I think this is dried DEF 'fumes'. I suspect the 'flush and fill' procedure is to make sure this residue does not build up in the various tubes and fittings. But .

I also suspect that the DEF systems in large diesel trucks are not as 'complicated' as the multi-tank system BMW designed with hidden tanks and lots of corrugated tubing, several pumps, etc. These on the road trucks would probably fill their tanks pretty regularly and hence their DEF systems would probably not get as 'crusty' as a low mileage 35d. But once again

Funf Dreisig

Edit: The active tank is visible in the first photo of this 35d after an accident.