On some versions of android if you long press on the popup it will show you which app it's from. This obviously doesn't work for not-android and older versions, though.

On Mac it is so easy to steal a password. In Javascript: var gmailPw=prompt("Facetime requires your password to login"); Macos asks it every week/month for one reason or another, people are conditioned.

My thoughts exactly. Just close the popup and open any Google app to verify that you cannot access the app without signing in again.