77ms of latency is pretty slow. Based off that I’d assume (but not rule out) that it’s not: on the machine you used to run nmap, not on ethernet, probably wifi with a shitty connection

So, some really dumb, likely irrelevant, questions that might spark an idea:

• Do you see anything weird connected in the wifi client list?

• Are you running a VPN server or using VPN to bridge any networks?

• You said you’re running dual WAN, are those configured properly and not leaking random internet shit into your LAN?

• Do you have anything that might be running some kind of out-of-band management system like DRAC on a dell server?

• What’s your IoT situation?

• Do you have an on-site NVR for security cams?

• Did you find the mac? If so what are the first 3 octets? Even if the vendor can’t be found, there’s always the chance some crazy ubernerd is going to recognize it. (If it’s 00:d0:2c or 44:d9:e7 I got ya covered)

Again, most of those are probably irrelevant, but throwing the thoughts out there :)