Find a file that's stuck on a branch and not been merged (and / or has been deleted)
This is a pain and something that can easily happen. Specifically had it with a readme file with some important documentation that we didn't want to, as a team, try and re-document.
This was the most "pragmatic" (ergo fastest to find what we were looking for)
git log -p --all -S 'search string'
git log -p --all -G 'match regular expression'
These log commands list commits that add or remove the given search string/regex, (generally) more recent first. The -p option causes the relevant diff to be shown where the pattern was added or removed, so you can see it in context.
Having found a relevant commit that adds the text you were looking for (eg. 8beeff00d), find the branches that contain the commit:
git branch -a --contains 8beeff00d