Asif, if user just closes the browser without loging out, he stays "online"
under "Members" sections as well as in chat for 30 minutes.
30 mins timelimit is set here: ow_system_plugins/base/bol/user_service.php:
method - deleteExpiredOnlineUsers
So after 30 minutes system checks whether user was active or not, if
user stayed inactive for 30 minutes ( and he surely did, since he just
closed the browser), CRON will "offline" this user.
Please check how long your users stay "online" when just closing the browser. If it is way more then 30 mins, let me know.