As you type, it will try to match with heading names and highlight matching ones (bold, italic and orange). However, this only matches visible headings; it won't look inside at nested headings that have not been expanded, or at the content within a heading section.
Click the search button to submit a search. This runs a full search against all headings, and should produce more comprehensive results. It searches both headings and the content immediately under them for the entered keywords.
Above: Full search with results appearing on the right hand pane
The search uses an 'AND' operator, and also searches content within a heading as well as the heading itself. So in this case above, the three sections contained the words 'user' and 'access' within either the heading and/or the content immediately under the heading.