I've noticed that the "seen" in chat is different from the "seen" in user profiles which changes when the user refreshes the page. How does it work for chat?
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A user is marked as having been "seen" every time a chat window open in their browser polls the chat server. This is a bit misleading, since oftentimes the user has not actually interacted with that browser window for a very long time. Having been "seen" essentially just indicates that the user's client is connected - it has very little use for determining if a user is around. To figure out if a user is available, the "last talked" time is much more useful.
There are two different servers for chat and for Q&A . When you come to chat room whether you talk or not but server gets ping that you came for chat and your time to come to chat is stored. So it indicates that you were active before time.
Same with Q&A server; It stores your ping time and shows you your active time for the current Q&A site.