@dgerety,

Do you happen to recall what the LED indicator status was when the default SSID was broadcast? 

When the Default SSID is shown, it is usually an indicator that the Node is in setup mode and not connected to the rest of your network. For a device to connect to the main network and connect to the internet, it would still need to be given addresses and credentials from the main deco unit - which wouldn't be possible if the device is in setup mode; especially since the problem deco did not have the credentials for the network available to it (or else it would have resorted to a wireless backhaul).

As an FYI, you may be surprised by some of the clients in the list - such as other decos or switches possibly appearing depending on your configuration. The extra devices could even be mobile phones or laptops set to connect with a Randomized MAC address.

As the devices have been connected, you will be able to see the MAC addresses of the devices in your client list. I would recommend attempting to look up what type of devices these are by using a MAC address lookup.

As it is unlikely that anything could have happened and so that you do not have to change your network password, I would recommend making sure that all of your phones and devices have Randomized MAC addressing or Private Addressing turned off so that your network can accurately track clients. This way you can determine if any new devices are connecting without your permission.