from Hacker News

I used Tinder many months ago in another country and had deleted the app.

When I reinstalled it in an entirely different country, I was shocked to see it open straight into my previous user account.

I was on a different phone and wifi network, had cleared my Safari cache, and I don't use Facebook or anything that could be leaking a shared login.

How did Tinder know it was me? Where does it store data that persists after deleting the app? What do they use to identify the device?