I installed some development environment which created a postgres user account. The user is actually an OS X user account with the name of 'postgres'. This is different from a user account within postgres.
I don't remember what it was, it was some time ago (perhaps MAMP, but I don't know whether MAMP even supports postgres).
Anyway, this was later uninstalled using the provided uninstaller.
Now when I log into my Mac, the postgres user always appears. But there is no trace of it in System Preferences ยป User Accounts, and there's no entry for it in /etc/passwd.
What is prompting OS X to list the user account in the login screen? I'm struggling to figure out where else to remove it from.