Theoretically, there could be hardware/software running on the device that learns the source/destination IP addresses of frames passing through it and then assigns itself a suitable static IP so you wouldn’t see it in your router’s DHCP table and then phones home that way…
Theoretically. You could try to just plug a single computer into it and run wireshark and see if you see anything. Theoretically its possible for it to not broadcast or send its own traffic until a gateway IP is identified and learned but this would be highly theoretical stuff.
I think all in all the effort to create a compromised stealth switch like this would GREATLY outweigh the effort required to create a working switch with a trusted brand name in a crowded market segment. But hey anything’s possible when you wear tinfoil on your head.
You can call them CPE or Customer Premise Equipment.