As you all know, even if WhatsApp is for mobile phones, we can access and existing WhatsApp account using the web version. In the beginning the service was available if your mobile and the desktop is in the same network, but now can be available anywhere on any device using the link http://web.whatsapp.com.
This feature was very useful mainly for those who are using laptop or desktop all the time. Even during the work, we can check and send messages through from the browser itself, we dont need to look in to the mobile all the time. We are also able to receive notification within the browser. However it is a good feature, there are some downside also, most commonly the account of one person can be monitored by other people.
In WhatsApp, we get maximum security, so that there are less chance of your account being hacked. But here the problem occurs knowingly or unknowingly. In some case we may give our phones to other people of some other purpose, but here, what they can do is just open web version of whatsapp on their mobile or desktop and can scan and login your whatsapp account on their mobile. So they are able to read and send your whatsapp messages from their device, untill you logout the device from your whatsapp account.
So, if you need to check your whasapp is beign logged in at any other location, please go to whatsapp settings, then select WhatsApp Web, If yo are seeing the screen like below, its fine, your account is safe and not logged in at any other device. Its currently showing a scan window to start loggin. So its fine, you are safe.
And if you are seeing the screen like below, you can see the list of all the logged in device, please check the login time and device, to confirm its you or someone else. If you found anything suspicious, click on the Log out from all computers. You will ask for confirmation, click on Log Out.
Now your account will be logged out from all other device and you will see screen like the first. So you are safe now.
Do check this regularly or if you find anything unusual. If you are accessing the web version from Internet Cafe or any public places, please remember to logout like this.