For about 30 minutes WhatsApp web stays logged in after you close tab and once you open the web again, it will be able to access WhatsApp if your phone has an active internet connection. Such an alert is shown when someone is spying your WhatsApp through the web.The WhatsApp web activate notification means that your WhatsApp account has been logged in from another device. Usually, such notification appears when someone is spying on your WhatsApp through the web. When you do see such a notification, and if it is not you who has logged into your WhatsApp account via your laptop, then it can only mean that someone is viewing your WhatsApp.