Search API is limited to known entities. But what if you have a Panels Page, or a page created by hook_menu()? This module gives you a way to enter custom paths where the contents can be indexed by Search API.

This module creates a new entity called "Custom search path". Add it to a Search API index just like Nodes or Taxonomy terms. On the Fields page, check the "Page content" field, and optionally the Title field. Now begin adding new custom search paths, and they'll appear in your index.