chance upon

If you happen to chance upon that document, let me know.

如果你碰巧发现了那文件，请通知我。

I half expect to chance upon that house, as would happen in a dream.

我只是期望我能见上一面那记忆里的老屋。

However it is non official and will be known to people who chance upon it (for now).

然而，它（blog）不是官方的而且将会被碰巧遇到它的人所熟知（对现在来说）。

On the other hand, we could with very good luck chance upon a signal tomorrow.

话说回来，如果交上好运气，我们明天就可能碰到一个信号。