If you want to be really sure a piece of software is GPL, it would probably be alright to include a file in your source tree on your harddrive saying that whoever finds it is free to make a copy of the project for themselves and consider that you have distributed it to them. It's about the clear intent to distribute.
That's a logical way of looking at it, but it disagrees with the GPL FAQ. Note especially "If the version has been released elsewhere, then the thief probably does have the right to make copies and redistribute them under the GPL".
Plus, you can have a unilateral contract that's open to anyone willing to accept the terms. You don't have to take any action per person to 'establish' it.
However I was wrong about contracts vs. licenses. Obviously I am not a lawyer.
http://www.groklaw.net/articlebasic.php?story=20031214210634...
The point is that for a license to be binding it has to be offered willingly. If I sign a blank check and you steal it from me, filling out $1000 and your name, although you'll be able to cash it, you have no right to the money, and I can undo the action.
The reason internal code is special is because the employees are not considered individuals to whom the company has distributed the software. Instead they are a part of the company.
I identified some relevant questions in the GPL FAQ. I believe that the first one and third one are what you care about.
https://www.gnu.org/licenses/gpl-faq.html#StolenCopy
https://www.gnu.org/licenses/gpl-faq.html#GPLRequireSourcePo...
https://www.gnu.org/licenses/gpl-faq.html#InternalDistributi...
https://www.gnu.org/licenses/gpl-faq.html#DistributeSubsidia...
I hope that helps. The FSF might respond if you write to them, it's worth a shot if you're still not convinced.