Searching for a solution gave me the following:

• How to find local git branches that have been deleted from master – I don’t think this answers my question, but if it does let me know. This question was only about the branches merged into master. My question is about any branch I have locally that is no longer on the origin (remote) git server.

• Remove tracking branches no longer on remote – This is very close to my question, but this answer deletes the branches. I’m interested in knowing what would be deleted if I ran git remote prune origin.