Make sure that you have accounted for all the devices that are connected. Some forum users have found that it is often devices that they forgot to include such as powerline adaptors or DVD players, smart TV etc.
When you are checking for the devices that are connected to your network, if you click on the device it should open another page and it will show you the MAC address of the device. You can then do an Internet search of that and it usually gives you the manufacture's name and this may assist in identifying it.
