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My own experience is I have seen the media partners bot hit just seconds after a visitor hits the page.
Mine too. In fact, this caused some problems - I have quite a few pages which look like...
...so as a result, I had two e-mails, one from the real user, and then another one from the MediaPartners bot!
I removed the ad code from that page anyway - it wasn't the best place to put any ads - and the problem ceased.
When I was uploading a few more files after I put the adsense code, I noticed there were a few visitors. Since this site was not used and not completed yet, I was not expecting any visitor yet.
But there were a few visitors following the pages I visited after uploading the contents to check it it looks ok.
I tried to scare off these visitors for twenty minutes by changing file names, etc. not to show my work-in-progress site.
Finally when I typed in the IP address in google.com, I found that those were the Media bots.
I was fighting with bots online...Funny..but real story.
Anyways, I now know that the media bots visit a page if there is a request for adsense from the page.