Strictly speaking, ZIP codes are not geographical areas. They are a construct used by the Postal Service to define carrier routes. While many ZIP codes can be approximated using geographical boundaries, many are points on a map. They could be Post Offices, multi-family dwellings, or a University. The US even contains one “floating ZIP code,” a boat called the J.W. Westcott II, a boat that delivers mail to ships on the Great Lakes as they pass through the Detroit River.
A ZIP code that doesn’t render on an Emsi map as a shape is likely a “point” ZIP code with no geographical area associated with it. These ZIPs may still contain establishments, employment, and population, depending on what kind of entity the ZIP represents.