Sure will be one or the other when "Googling", especially in the "Images" search mode, noticed that contained oddly images and texts of access artillery pages in the search index. These results are notable in that it is not on the target page without previous user account or login.
But like the Google bot then came to the content, yes, finally he has in the search index?
The solution are HTTP headers, such as the browser ID to identify the different Google bot as bots and from normal web surfers. Some site admins have, therefore, a browser switch tinkered certain bots to pass through, but required by normal web surfers an application.
This makes the content of a site for the Google bots available without in the same for the public release. Background for such behavior is, of course, that some sites owe their popularity to their search engine findability, respectively, but they generate their revenues through subscription, pay-per-view or similar models, which definitely require a user login.
But even for those Browerweichen there are elegant ways to work around them as a normal user. Surely you can manipulate the HTTP header of the browser directly, which means Firefox extensions even goes pretty easy. The course assumes some knowledge and some handles.
Better because of the way through a web proxy such as Be-The-Bot , this should not change anything on the browser and access to certain sites are "more or less" anonymous through the proxy.