you just need to add UID to
Application object also so that this can be accessed out of your session also.
I guess strUserid is unique and a string ( if strUserid is a number then convert it to string for adding to Application object) follow these steps
Session.Add("UID", strUserid);
if(Application.Get(strUserid)!= null)
{
}
else
{
Application.Add(strUserid, "LOGGED_IN");
}
also when someone logout then remove the UID from Application