Original Brake Rotor Observation
 

I just noticed that on my 2009 C63, the front rotors are cross drilled, but the rear is cross drilled AND gas slotted.

Is there a chance that the previous owner changed the rotors on the car? Got it at 8,000 miles, so I would not think they would need to...but its kinda odd that the rear is gas slotted while the fronts aren't.

No, that's how they are from factory with the 'standard' brakes. Don't worry.

This is gonna sound stupid, but look again at the fronts. They should be slotted, but with only 2 or 3 slots. If the wheel is sitting at certain angles when you parked, you will miss the slots. I think that b/c the rear rotors sit further back behind the wheel, the slots are easier to see.

My 2010 is slotted and drilled at both front and rear (not a P30/P31 car, either). The '09s and '10s should be the same.

I will have to take another look, but I don't recall seeing slots on my front rotors. My E63 does, not sure about the C63. The rear rotors are slotted and drilled, to reduce excess heat generated when the rear brakes act in the capacity as a Limited Slip Differential. Since there is no LSD installed in the standard C63 without the Performance Package, the rear brakes will clamp down, on the wheel that is spinning, thereby transferring power to the wheel that isn't in order to keep the rear end in line.

Maybe this is a better explanation "Due to the tail happy nature of the C63 the Electronic Stability Program (ESP) is in overload with trying to keep the wheels from spinning by applying brakes to the rears as needed thus generating a lot of heat that needs to be cooled down."

totally off topic but i wonder if you conserve brake use and heating issues if you run your car in ESP Sport to maybe limit the rear brakes applying themselves.