Is location and IP tracking being used instead?
The past 12 months, we've seen a big increase of tracking and targeting by IP address instead of browser. So in a multi-device, multi-person household you get targeting 'leaking' between the devices, some of which are locked down and some which are generally open to being tracked. For instance if my daughter browses for jewellery that then seems to result in ads and recommendations appearing on other devices in feeds and suggestions. It also seems there might be some leakage from neighbours - things they are interested in that we're not - based on location.
The risk is that if Google or other tracking company monitors say visits to family planning sites, or health advice sites, or unemployment advice, the system could easily leak this information inadvertently to family or neighbours simplying by displaying the wrong ad to the wrong person.