Have you looked in your service book? The code is usually on a credit card sized chitty on the inside of the front cover!
I've had the battery off a few times and it has never asked me for the code
The stealer should be able to get the code for you, or re-code it.
They will charge unless you soft soap them a little
Some audio dealers have the capability of re-coding the head unit - just like some people will re-code mobile phones
