Curufinwe, I was thinking about your question and wanted to explain...
The chat program we use at the bottom right is by a third party called envolve – it's not very good and I'm looking to replace it – but separately to that, it uses a different way of detecting who is online and how many users are online at any given time.
The forum (which I wrote), where it says "and is online" above each user's post, uses the "session" store of our web server. Basically, every time a user visits the website a session cookie is created which I store in a folder on the server and associate with that user in the database so I can detect whether a logged-in user is online or not. The server, periodically expires these sessions, however, this isn't instant and often can be 30-50 minutes in between. This basically means that the forum can sometimes report that a user is still online, when they're not – and this will be cleaned when the session expires.
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