[mverwijs]

Alternate explanation: the person(s) you discussed this with did some google'ing and browsing. The tracking cookies do their thing and since you and those persons are linked, you end up with the ads served.

[cryoshon]

a good alternative hypothesis, but also one that should be proveable, at least in some cases.

designing an experiment to test this would be quite easy.

ultimately your explanation could exist alongside audio snooping, perhaps even helping to resolve areas of ambiguity...