Hmm. That would mean it's likely one of the following (well perhaps more options, but these spring to mind)

• A windows machine that has the network set as a public network, or netbios specifically blocked on LAN.
• A windows machine that has all the netbios services disabled.
• Not a windows machine, or a container as others suggested that's running some kind of IIS install
• Not a windows machine at all but for some weird reason IIS files and a web server setup.

I think you suggested in another comment, that it's not in your DHCP client list but has an IP in your normal range. Which suggests it is setup with a static IP. That is odd.

Some other people suggested it could be a container that is using a real IP rather than the NAT that docker etc usually use. I do know that you can use real IPs in containers, I've done it on my NAS to get a "proper" linux install on top of the NAS lite linux that is provided. But I would have expected that you'd know about that, since it would require someone to actually choose the IP address to use.

If you have managed switches you could find which port on which switch the MAC address (as found by lookuping up the arp record for the IP using arp -a) is on (provided the switch allows access to the forwarding tables). Of course, if they're on Wi-Fi it's only going to lead to the access point they're connecting to.