Guess: your friends also took pictures of the same events from similar angles and uploaded them, fb worked out their location from one of the other suggested me

Guess: your friends also took pictures of the same events from similar angles and uploaded them, fb worked out their location from one of the other suggested methods, and then matched it to your photos through simple computer vision algorithms. Or, you used facebook at some point while in that city, facebook recognised that your photo was of a baby just born, by comparing it with the millions of other photos tagged with "OMG MY BABY IS BEING BORN!," it knows that babies are born in hospitals, and it knows that there is only one hospital in that city, so it guesses that the photo was taken in that hospital.