I'm the same...
I have my RCD510 sitting under my desk just now waiting to sell, but have no code!
It was in the car for less than a day and is immaculate.
There was a number scrolled in the front of the manual, but I wasn't sure if this was the code. The dealer told me that they no longer supply the units with codes as most people leave them in the glove box - which made them easy targets for thieves.
I asked the dealer to get my code. I needed to give him my VIN number and the serial number of the stereo.
I'm still waiting for him to come back to me with the code though!