Hiya Simon! 
I asked our chat team and they shared this note from the library that handles the user presence:
Which equates to: show online if…
- browser is not in the background, and
- there has been user activity in the last minute
Both are browsers, and allow user activity, so it will reflect it when the user is actually looking at the site (merely opening Discourse Hub app doesn’t constitute “viewing” the site in a browser, for instance).
Does that give you enough detail to share with your community? 