Basically the only 'connection' between an open page in the browser and server, served it is the session at the server and some id the browser manages internally to be able to re-connect to that session...That session at the server there is a life-span (20 minutes by default in IIS) that will comes to and end with an event - Session_End - in your global.asax...
https://msdn.microsoft.com/en-us/library/ms178583%28v=vs.140%29.aspx[
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Setting the status to offline at that event may give you a solution...