Comment by philsquared_

You have hit the nail on the head but it doesn't even need to be wifi and it also doesn't have to be that complicated. They see that two devices are in the same area for a long period of time so they serve ads for other peoples web history. Jarringly enough ever since I had this realization I sometimes see ads for things that reveal something meant to be private of those I had just spent time with. Also, seeing peoples ads when they make desktop recordings of their screen can be extremely telling..