I have a blog that I used to show Adsense on, but with some Mature/Adult content, I removed Adsense and now show Black Label. After only a few weeks I get an email from google asking why I stoped.. So it just goes to show, that they are watching, and that the big G see all. Even if you don't use Adsense.
I get about 450 UV per day, and about 40K page views per month.. My Adsense account in pin verified, but is unused at the moment. Maybe I will use it on another site if I get a chance.
It is all automated emails. Read in this forum, people receive them for a variety of reasons and many of them think those messages are specifically made for them. They are templates messages with sometimes placeholders to show your domain name or even an example page of your site.
I know it's all automated, it was just the responce time that caught my eye... within two weeks of droping Adsense the email was sent.. Which make me think churn is a problem for them.