To chance upon something is to meet or find it by pure accident. Chance upon an old friend.
The accident is the point, since nothing you were looking for can be said to chance upon you. It is slightly literary, and come across is the everyday version.
1phrasechance uponTo meet or find by pure accident.
“He would chance upon the same man in three different countries.”
chance upon is a phrase.
Practised in Phrasal Verbs