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.
happen (up)on someone or something
to come upon someone or something, as if by accident. (See also (up)on someone or something">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.
happen on
or happen uponv. 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.