Found my own one in the wild today!

Grey cable goes to the switch, brown disappears under the floorboards, likely means that this goes to some kit that we don't manage. Couldn't log into the switch to check the MAC / IP address for more investigation. Noted it on the customers documentation and reported to our contact on site who said everything is working, no-one has been in there for ages so wasn't concerned.

I happily moved on as it's not my dumpster fire in the making! (I work for an MSP). It will likely become my problem in the future but at least I have documented to cover my ass!