When my PC running Ubuntu 15.10 boots up, it goes to the login screen there are two more users. The two new users are "postgres" and a Guest Session I have not recently indtalled any new programs.
1 Answer
The guest account is added when you install Ubuntu.
The postgres user gets added when you install the postgres database. I would assume at some point you did install postgres. If not you need to examine you system as it could mean someone added software without your consent. Postgres gets installed with some other server related software though (like Jasperserver) so could be created when you installed that.