Happen on
Definition 1 of 1
Pro Tip 1/3Accidental Discovery
Used when you discover something or someone without planning or expecting it. 
On our hike, we happened on an old, abandoned mansion.
Pro Tip 2/3No Prior Intent
Implies no previous intention to find something, it's all by luck or accident. 
She happened on a rare book at the thrift store.
Pro Tip 3/3Unplanned Meeting
Can refer to unexpectedly meeting a person while doing something else. 
I happened on my old teacher while vacationing in Greece.