If your Smartphone is in hands of your friends and family members than within 15 seconds then they will be able to hack it.
They will just open their laptop or computer and will open WhatsApp on browser, it will show a QR Code. By scanning this QR code on your mobile they can use your WhatsApp through their browser.
If you think someone is seeing your account through web then just open you WhatsApp and go on to the options. In the options you will see ‘WhatsApp on web’ click on it and it will show you the last time that your account was used through web.
You will also see an option ‘ Log out from all computers’ by clicking on it your WhatsApp account will be logged out from all other computers.