Chance upon
Definition 1 of 1
Pro Tip 1/3Unexpected Discovery
Used when someone discovers something they weren't looking for. 
I chanced upon an old friend in the market.
Pro Tip 2/3Positive Surprises
Often implies a pleasant or beneficial find. 
We chanced upon a quaint little cafe.
Pro Tip 3/3Casual Tone
The phrase has a casual, conversational feel, not very formal. 
They chanced upon a deal while shopping.