Federal jobs that do not get posted on job listings are called excepted service positions, and these jobs can only be found by contacting the organization itself. Organizations such as the CIA or DOD tend to have their own requirements and laws for employment and cannot always post their open positions on public listings for security reasons.
Various courts and political organizations that help manage the United States mostly hold excepted service positions. In the case of political organizations, a lengthy election process may be required. These organizations have many restrictive laws about who can run for election. In the case of courts, a great deal of education and experience is required before a candidate is considered, and many of the positions also require votes.
The United States Postal Service only posts jobs on its official website, but it has less restrictive conditions for applying for jobs than other government organizations. A post office worker might only require as much experience as someone applying for a similar job in a private company. Additionally, the USPS offers jobs and programs to new graduates, and former members of the US Armed Forces are given special consideration for jobs in the postal service.