Have you tried cleaning up an AdBlue spill that's had time to dry? I apprenticed at an auto shop before I switched to metalworking and I had to deal with a number of AdBlue messes. Newer cars make it easier to refill, but the location of the AdBlue cap in some older models makes you rummage around the luggage compartment.

God forbid you have a sensor problem with the SCR system since your modern diesel is programmed to be crippled if it reads there's no AdBlue in the tank. We had a car come in once (a Citroen or Renault I believe, I remember it was some sort of Frenchie) that needed its rear left fender to be removed to replace the damn thing and I'm 100% sure devils designed the way everything attached together.