Although Google crawls billions of pages, it's inevitable that some sites will be missed. The real challenge, more than making the overall system easier to use, is to find those elements that weren't previously covered by the search engine.

In the context of defense and security, sites can be missed for three reasons:

Poor timing. An online activity may not be public knowledge, but it can be shared between network administrators. In other words, links are exchanged between sites, for instance between military sites and different public networks.