Can Drupal's search index external tables? Can Drupal's search system index external tables?
I have looked up how external tables can be linked to Drupal using the Data module, hook_views_data etc., 
Can the content of the external tables be indexed and searched by Drupal's search system?
Yes, you can use the search api to specify your additional content. In particular you will need 


hook_search_info 
hook_update_index 
hook_search_execute