we all know this… piratenpad.net (etherpad), doodle.com, dropbox etc. all generate long and cryptic links that we can share to allow other people to access specific content.

maybe i am mistaken, but that is what google says:

„It’s almost impossible to keep a web server secret by not publishing links to it. As soon as someone follows a link from your „secret“ server to another web server, your „secret“ URL may appear in the referrer tag and can be stored and published by the other web server in its referrer log. Similarly, the web has many outdated and broken links. Whenever someone publishes an incorrect link to your site or fails to update links to reflect changes in your server, Googlebot will try to download an incorrect link from your site.“