Surely you used several USB Ethernet adapters to rule them out as being the source as well right? Those types of dongles are well known for calling home.
Good observation :) Multiple ethernet adapters: Apple original (ancient USB2 10/100), Tier 1 PC OEM, plus a few random ones. Some USB adapters emit more RF than others.
It excluded the published hostnames for services and CDNs (some of which resolved to GCP, Akamai, etc) published by Apple for sysadmins of enterprise networks, https://news.ycombinator.com/item?id=46994394. It's indeed possible that one of the unknown destination IPs could have been an undocumented Apple service, but some (e.g. OVH) seem unlikely.