I saw you with your friend in Shibuya just by chance.
I happened to see you and your friend in Roppongi last night.
I saw you with your friend in Shibuya just by chance.
「君が友達と渋谷にいるのを偶然見かけたよ。」
I saw you .... just by chance. で「偶然見かけた」と言えます。
I happened to see you and your friend in Roppongi last night.
「昨晩、君が友達と六本木にいるのを見かけたよ。」
I happened to see you でも「偶然見かけた」と言えます。
happen to で「たまたま…する」という言い方です。
上の文の最後に but you didn't see me をつけて言っても良いですね。
I happened to see you and your friend in Roppongi last night but you didn't see me.
「昨晩君と友達を六本木で見かけたけど、こっちに気がついていなかった。」
ご参考まで!