happen on

happen (up)on (someone or something)

To find or encounter someone or something randomly or without planning or intent. I happened on a quaint new craft store the other day. I happened upon your brother yesterday in the grocery store.
See also: happen
Farlex Dictionary of Idioms.

happen (up)on someone or something

to come upon someone or something, as if by accident. (See also come (up)on someone or something.) I just happened upon a strange little man in the street who offered to sell me a watch. Andrew happened on a book that interested him, so he bought it.
See also: happen, on
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs.

happen on

or happen upon
v.
To find or meet someone or something accidentally: She happened on a $10 bill that lay on the sidewalk. The detective happened upon a clue while searching the cabin.
See also: happen, on
The American Heritage® Dictionary of Phrasal Verbs.
See also:
  • happen (up)on (someone or something)
  • happening
  • happen by
  • happen along
  • a whole nother story
  • nother
  • be taken by surprise
  • arrive at the scene (of something)
  • straightaway
  • save face