Ask HN: How has Facebook figured out my family doctor as a friend suggestion?

I never use Facebook on my phone so you can hopefully rule out contact list networks - not that I ever had their personal mobile on my list - and location tracking because I haven't attended my doctor's surgery in years. We have zero friends in common. Other than my doctor actively looking me up on Facebook - highly unlikely - how on earth is this possible? I'm willing to accept friends of friends suggestions but this is beyond spooky.