The story of how Target discovers and targets recently pregnant women is a good example [1].
An interesting quote: “we found out that as long as a pregnant woman thinks she hasn’t been spied on, she’ll use the coupons. She just assumes that everyone else on her block got the same mailer for diapers and cribs. As long as we don’t spook her, it works.”
There's no reason to believe Google isn't doing the same thing. And I strongly suggest reading the original article[2], if only for the first two or three paragraphs.
