Chance on
Definition 1 of 1
Pro Tip 1/3Unplanned Discovery
Use 'chance on' when describing a surprising discovery made without a plan or expectation. 
While hiking, we chanced on a secluded waterfall.
Pro Tip 2/3Casual Encounters
'Chance on' often implies a casual or unexpected encounter with a person or thing. 
In the library, I chanced on my professor.
Pro Tip 3/3Serendipitous Tone
The phrase conveys a positive tone, suggesting a fortunate or happy accident. 
She chanced on a rare first edition at the flea market.