Coolant

I have a cat c15. No def. Twin turbo.
I dont loose any coolant.
You have a leak somewhere its just not one you can see. Its probably a small leak, probably leaking out when your driving and the heat of the engine is evaporating it. Or it might be leaking out the overflow while driving.
Cheapest thing to try is a coolant cap. If thats weak it could be letting coolant leak out while driving.

 

Check the pre cooler. Take the tube from the turbo to the charge air cooler off. If its wet then your pre cooler is bad

 

First step is to check the radiator cap make sure the rubber seals are good and it has spring tention . Then you need to pressurize the cooling system with a tester. Pump it to 15lb and see if the system holds that, look all around the engine for signs of it leaking.
It could be a coolant diverter hose weeping, they are hard to see.

If you don't find anything by doing that search this forum for the bottle test, is been explained on here quite a few times, it's a test that will show you if you have a bad head gasket or low liner height issue.

The cats are prone for chewing on coolant for a little Whyle before they let go properly.