Filter the_posts doesn't work on search page

Question

In a plugin, I have this simple code:

add_filter( 'the_posts', 'process_search_results', 10, 2 );
function process_search_results($posts, $query=false) {
    print_r($posts);
    return $posts;
}

This shows the list of posts that will be looped over in the loop. But on a search page, an empty array (Array()) is printed, while the search results are still shown. Why is $posts empty for searches?

Some background: I'll try to code a custom search plugin that should merge results from an external data source into the default results.