I think you vastly overestimate the organizational abilities of Google recruiters. I have friends who, while working at Google, had recruiters contact them to ask if they were interested in working for Google.
I'd say listening to what someone is saying and assessing whether they might be more interested six months later requires far more sophisticated thought processes than looking up whether someone already works at Google before trying to recruit them -- so making sure that you're very explicit about what you want them to do (mark your file as 'do not contact') is probably a good idea.
but you know, google's mission is to "organize all of the world's information". I assume this includes HR info as well. With all of that data and all of the high-end machine learning talent that works there, you'd figure that they would be able to learn when someone has no interest working there.