[camaroguyBC]

to be honest i don't think that's a marker.

i have almost the exact same thing on mine in 2 locations on the left rear wheel. when i was washing the car last weekend, i also had some up on the side of the car just behind the rear wheel (in spite of the splash guards).

it seems to be some form of grease / oil / tar. it won't come off by hand, and it takes a TON of scrubbing with regular car wash stuff to make a dent in it.

i whipped out a can of bug/tar remover and it came off really easily on both the wheels and the paint.

no idea for sure what it is, but again, it looks identical to what you have in those photos. it's not limited to the camaro either - i had the same thing on my 2006 Saab 9-3 on a couple of occasions. I'm guessing it's not from the car itself, but perhaps something on the road (like tar in the cracks on the roadway or something that liquifies when warm).

good luck.