Storing ..um.. classified Information on your webserver in hidden directories is not exactly very bright because you can be sure that google will index anything it can get.
The goolag scanner, which was recently released by the famous hacker group cultDeadcow actually found pornography on chinese government owned (gov.cn) sites and security holes on western servers.
http://www.cultdeadcow.com/archives/2008/03/cult_of_the_dead_cow.php3
Please assume that files being stored in http://mydomain.com/_this/is/a_super/secret/directory/ will easily be found by simple methods such as specially designed search queries:
intitle:index.of +”last modified” +”parent directory” +(pdf|doc) +”Secret” -htm -html -php -asp