I know you said "ethernet" was unplugged, any chance WiFi is on and feeding the receiver internet?

I had a very similar situation with a Yamaha receiver... Turned out to be the customer's kid streaming Spotify connect to the wrong destination. Kid was at college, tried to stream Spotify to her family room but was actually streaming to her parents family room.

Simple solution was to disconnect the receiver from the internet. The parents weren't using any internet features, so no big deal turning that off.

Problem solved.