That was my thought, until Spike mentioned Apple.
I wondered if, perhaps, one of his devices had updated / reset and was
sending two SSIDs ( not unknown) or it could be a neighbour doing the same.
Even (some) smart plugs have a mode when they set up a “weak” local network
you connect to, enter the details of your secure, real, network, and it
stores them for future use. I don’t recall seeing a smart plug ever
transmitting it’s own network again- unless put into the mode- but I had an
access point which seemed to have a mind of its own and would transmit the
SSID I set up plus the default one from time to time. You could connect to
it, with the pw for ‘my’ SSID. ??? A replacement did the same.