Messenger also has the option of end-to-end encryption using 'Secret Conversations', encrypted using the Signal Protocol.[1], [2]

Is there any indication that Facebook doesn't scan the contents of messages prior to encrypting them with your keys and sending them across the wire?

[0]: https://www.bloomberg.com/news/articles/2018-04-04/facebook-scans-what-you-send-to-other-people-on-messenger-app [1]: https://www.facebook.com/help/messenger-app/1084673321594605?helpref=uf_permalink [2]: https://www.wired.com/2016/10/facebook-completely-encrypted-messenger-update-now/