What steps have you taken so far to try to identify the additional devices?
Have you eliminated/accounted for all IoT devices (such as heating controls, domestic appliances, AV equipment etc.)?
If you have any powerline devices, we have seen before on here powerline devices from adjoining properties appear on the hub interface when default settings are left in place and some aspect of the electrical fixed wiring is shared between properties.
If you are using Apple devices, see if this past similar topic helps
https://community.virginmedia.com/t5/Forum-Archive/Hub-3-Rogue-5ghz-WiFi-Client/m-p/4898504#M451335
