The search engine does not implement any access rights filtering for documents (pages, files etc). However, when using the EPiServer CMS integration, the return value from an extension method for PageData named RolesWithReadAccess will be indexed. This means that we can filter out pages that the current user should not be able to see.
Filtering out results using RolesWithReadAcess:
SearchClient.Instance.Search<StandardPage>() .For("Possibly secret stuff") .Filter(x => x.RolesWithReadAccess().Match("Everyone")) .GetPagesResult();