What you described is basically how it works. You all get finger printed on the same network, finger printed on google searches, finger printed at the routers, and then associated. 6 guys get together for beer and someone brings up wood chips, 1 guy later googles BBQs, you're all getting BBQ ads for the next week and a half, the 5 other guys think the phone is listening to them because they don't know one of the guys actually googled or triggered something related to BBQs. I once knew my roommate was pregnant before anyone else because I got a bunch of ads for baby shit all of a sudden.