It sounds like you are on the same network with another machine, named "sagemcom", that is acting as a file server (or offering some other network service).
If you go into the Finder, then choose Finder > Preferences…
- The General tab will let you choose whether to show "Connected servers" on the Desktop.
- The Sidebar tab will let you choose whether to show "Connected servers" and "Bonjour computers" in the SIdebar. (Bonjour is Apple's name for a group of technologies that make it easy for devices to network with each other without a lot of configuration.)
I suspect that the Network header in the Sidebar may only appear when the Mac sees other network devices which you might want to connect to, and when the Finder Preferences say to show these items in the Sidebar.
That leaves open the question of what is this other network device, and why is it on the same network as your Mac. But now you know what the Sidebar is doing.