I believe headers used by a source file appear in the Keil project list, nested under the source file. Clicking the + box, and then double-clicking the .h file would open the .h file.
In my project I don't see + boxes next to source files. When I search variables, I don't get any hits in header files because they are not in my project. I shouldn't have to manually add all the header files to my project. I thought header files automatically get nested in source files in the Keil Project Window.
Never mind! From project window, I righ clicked on source file, selected "Show include file dependency". Now I see + box next to all my source files.