Search only indexing public fields
Drupal renders a node and passes it to indexer. So whatever you see publicly that what is indexed. 

but there are few options you can fix that:


to render a node for indexing this is called: $build = node_view($node, 'search_index'); view mode is search_index, so in your node template you can check what view mode it is and render it differently.
you can use display suite module and then use this display "Search result highlighting input" to configure what is indexed
after node is rendered this hook is called $extra = module_invoke_all('node_update_index', $node);  you could implement it and return extra information 


hope it helps