Come upon
Definition 1 of 2
Pro Tip 1/3Unexpected Finding
Used when discovering something by chance rather than by searching. 
I came upon some old letters in the attic.
Pro Tip 2/3Positive Surprise
Often implies a pleasant or fortunate discovery. 
They came upon an old village that was perfect for their film.
Pro Tip 3/3Instantaneous Encounter
Implies a sudden or unexpected encounter with something or someone. 
Jogging in the park, I came upon a famous actor.