"Nomad said that most of the suspicious traffic didn’t escape the network. He has a hypothesis that the unit was simply affected by a bug. Nomad said he has over 10 years of experience in IT and works as a Technical Field and Community Specialist at the 3D printer manufacturer Prusa."
Absolutely what I thought. I love how everyone assumed it was hacked... In the 20 years I have been at my job we have had a particular model of desk phone that it happened to twice and when it did it creates a network storm and tanks your entire network (at the time this company did not have proper vlans in place which would have mitigated the issue). You trace it to the device and reboot it and everything is fine again. The phones were still replaced just in case, but that was always one of the wilder ones here. My guess is this was something similar and since it's his older parents they probably didnt even notice the network being bogged down.
"After rebooting the device and reconnecting it to a segregated virtual LAN (VLAN), the issue did not reappear."
Why even bother? Is it actually required to have an internet connection? If so get rid of it, thats so dumb and intrusive... and if not just turn off wifi on it, why even leave it on at all.