MAC Addresses
Seeing as so many have said that these won't appear anywhere on your normal packet:
How hard would it be to code an app, that when taking input from your local system also takes note of the the MAC address of your PC (there's a number of command line prompts that can show this afaik, so could not an app invisibly call one of these, or similar?), or even of your whole network, and then send them back to Google as, say, plaintext, to be cross-referenced?