Don't know about Mini per se but it is common that some areas of a car don't get fully covered when the car is painted (sprayed).
The first coat, primer, is applied by dippng the car in a vat of paint, but the final exterior color is most often sprayed on -- by an army of paint robots.
The areas that are visible say when the door is opened or the trunk or hood are fully painted (generally) but if you look at areas that are normally hidden behind plastic panels, or other outside sheet metal the exterior paint covering is generally pretty thin. There is just primer or whatever first layers of primer/paint are applied.
You can see a brief bit of this at this link: Uh, sorry the link is not present because I have not yet posted enough posts.
Do this: In a google search window enter the following search string (without the quotes) :
"factory building a mini cooper s"
You should see a hit: "New Mini Production - YouTube". Click on that link.
if you bring up the video I watched at around the 6:34 mark you see the row of spray nozzles apply the exterior final color.
At around the 7:06 mark you see individual robots apply more color. But notice some areas of the "tub" if they get any exterior color at all get it via overspray.
But there are a number of different videos. I haven't watched them all so I don't know if all show the same thing, more of it, or less of it, or something different.