1. Get addresses of mental care clinics and offices.
2. Geofence addresses
3. Correlate devices that visited geofence addresses (using LiveRamp data) with devices that saw your ads on Mental Health sites.
4. Bonus, look at the path on the pages to figure out what disease they were viewing when your ad was displayed if you weren't already targeting specific page content.
Healthcare targeting has been severely reduced in the past couple years because of concerns like these. You used to be able to target diseases/interest in diseases/etc but can't anymore.
("absurd" is a strange word to use; it might be nicer to educate without condescension).
Location data is so easily available that is largely a commodity now, sources include GPS from "always on" apps like the Weather Network but also apps that are collecting this data without your permission. Apple and Google are constantly kicking apps that do this out of the store.
Also tricks like "local wifi" devices are being used, Apple just reduced apps ability to sniff networks for this reason. There is another response that lists some other data sources, your cell phone company being the worst offender for many reasons.
> > Correlate devices that visited geofence addresses (using LiveRamp data)
LiveRamp is hooked into a lot of ad services and a user's location information will routinely make its way to them. Played some ad-supported mobile game on your phone while in the Doctor's waiting room? LiveRamp has that location data.
Even if the user doesn't have GPS enabled for the game, they might have it enabled for some other background process that routinely asks for it, like Facebook or Twitter or FourSquare, who package that data for sale. Or you're on the Doctor's wifi, and most stable IPs like that are in location databases. Or they just buy it off your cell phone provider, since many happily sell location information.
LiveRamp is a gigantic business built entirely on knowing approximately where you are any time you interact with their tracking servers.