Last week, I got hit by a car while riding my bicycle. You won't believe in how many colors an upper arm can shine. Anyhow, I took a picture of the arm, shared it to a friend on WhatsApp and promptly got a newsletter from Pinterest promoting tattoo posts. I do not have the Pinterest app on my phone, barely use it otherwise and would never search for tattoos since I'm just not interested.
I've been trying to find out since then where and how Pinterest might have gotten my blue/red/yellow/green arm picture from to analyze it, interpret it and link it to my account. They might be able to search my friend's phone's pictures (in case he's got the app which I'm not sure) and link the picture back to my account. Spooky though.