[srean]

Yes, as you would note, this was one of my hypotheses. However, it is on shaky ground.

This friend has suffered from vertigo chronically, it was not a new one off. My wife's and her friend's phones have been close proximity many many times before. It's certainly odd that Google would recommend vertigo only after a vertigo spell happened in the presence of my wife. None of the three searched for vertigo.

Phone motion sensors detecting a vertigo spell ? Well that's a possibility, but I doubt Google would be running such a detector 24x7, seems too expensive, unless the opportunity to show a timely ad is lucrative enough to cover the cost.

Although none of the three searched for vertigo, the friend may have searched for her pharmacy to refill her meds.

This is not the only incident. I have come to believe what I now believe about this eavesdropping, after a long period of whittling out competing hypotheses. I would usually file these incidents under confirmation bias. But these have happened just too many times.

A quantative Bayesian analysis would have been the right thing to do. On that count I am delinquent. I will, however, grant you this, human intuition is terrible at Bayesian analysis and tends to see significant patterns when there are none.