You notice this quite a bit on white ones, not usually as prominent on darker ones though.
My opinion is it's just a build up of grime. A bit of TLC will get it off but to do a really superb job and get everywhere you need to unbolt a few things to move them around and out of the way.
Front guard has clearly been off that car which of course is common at that age, but still, make sure you take a good look around it and make sure its straight and that the guard has only been off for cosmetic repairs, not structural.