VS asks for a key the first time you run it.

Since 2013 it doesn't even do that. It connects to your MSDN-account to check if you have the required licenses.

At least mine does..

Same. It punches through to an account with licensing on it. Specific VS licensing, too, as our MSDN account doesn't offer anything other than the installer unless we opt to buy seats.

Mine needed a key, I guess because I bought it off the shelf.