The phrase "to come upon" means to discover or find something by chance, without intentionally seeking it out. It can also mean to encounter someone or something unexpectedly in a place or situation.
It means that all property owners (whether the property is residential or commercial) must act reasonably in protecting people who come upon their property. (akivagoldman.com)
I don't know what I would've done if I hadn't come upon such a thing like this. (nintendotoday.com)
If you search thrift shops and yard sales like I do, you never know when you might come upon something golden. (shabbyfufu.com)