i have been wrenching cars for nearly 30 years, (not professionally), but have never seen one that after 13 years of use wasn't covered in grease, there is no way a 13 year old car has a dust cover that clean,

if not covered in heavy grease then road grime, mag chloride, just dirt in general.

that thing did not come off a car, especially a suspension or steering component

even the coolant recovery theory does not hold much water with me, most if not all do not have the hose run through the cap, the hose would be in most cases attached with a separate nipple that is a molded part of the bottle. and the cap does not thread on, it snaps on