Why are randoms finding my friends stream if he isn't promoting it?

So my friend just recently swapped from console to PC a month ago and he's not a very well-known streamer, I mean he's very very small, and he's noticed that recently there will be people from the games he's played in lurking in his stream. He will play stadium with people and he'll notice the same usernames of the people he just played with lurking in his chat afterwards. They don't say anything and it's not an issue to him, but he's wondering how they even find his stream because he doesn't go into team chat, he doesn't promote his stream, he's an average teammate and just plays with them just like anybody else.

So we're both wondering how they even find out he's streaming? On console if you went to someone's profile, you could see if they were live but he's not sure how it works on PC and frankly neither am I. Just a curiosity thing, thanks!