Every network card (device) in the world has a unique MAC Address. This information is transmitted with any data sent over the internet. That is why hackers destroy network cards / devices after a hack.
It would take less than hour to know if more than one device was used by determining whether their were multiple MAC Addresses. Network security 101. Any clown can determine this.
MAC addresses are generally not passed on by routers. The router keeps them in an ARP table and that can be accessed remotely if someone wanted to see them. But they would have to get into that router or into a computer on that subnet to see any MAC addresses.