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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.
happen onor happen upon
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.