Copy files off of a computer without leaving a trace

If they are on his local hard drive any bootable CD should allow you to do this. I'd probably do it with an ubuntu live CD. If you wanted to be really paranoid I guess you could dd the whole hard drive without mounting the file system so that it couldn't possibly write anything then mount the dd'ed image on a separate pc to do the copying.

"This is a slightly scary question. Maybe there is a way to get around the auditing controls, but you could still get 'shoulder surfed' by your boss.

It's normally better to simply 'tell' any new employer about your work, after all, if you have boosted your previous firm's code, what's stopping you trying the same trick on your new employer? That's what you should be worried about."