@MattH 

So it is definitely a Samsung device.

It could be a phone or TV.

Open the Deco app, if you see the device listed, tap on that, then select the gear on the top right. You should get more information about the device.

@Tony thanks for the reply. I have tried that but unfortunately it doesn't tell me much, just the MAC address

@Tony just tracked it down to a TV though, as found the MAC address in it's settings. The one thing I am confused by is that it shows up in Deco when the wifi isn't even set-up? Oh well, at least I have found the culprit, thanks.