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.