Correct !! I have one door like it - on the passenger side lolI can tell you what that is :-
tomo has sussed it, as it's where the woman in your life has placed her fingers after touching her face -- scent, makeup, persperation on the said fingers etc.
"Er Indoors" and I had a little tiff about it, not long after I bought the car.
Re cycled plastic - must be a reaction.