@billionto ,This is something I would agree on. A single Whatsapp account cannot be used in mutiple android phones. If someone somehow (if they get your sim with sms access that is) does activate and verify your account in their phone, then it would log out automatically from your phone.The only possibility is that: your whatsapp is being used on Web/Desktop. Go to your whatsapp settings/ Whatsapp Web/Desktop. There you will find where your whatsapp account being logged on. There is no other possibilities.
You can also choose to log out from all devices.