Wiki pages that begin with an underscore have special behavior, including two special case wikis that use specific filenames.
Underscores in Wiki Names
Wikis whose names start with an underscore (for example: "_hiddenPage") do not appear in the wiki table of contents, for non-admin users. For admin users, the pages are visible in the table of contents.
A wiki page named "_header" will replace the default "LabKey Server" logo and header. Adding the _header wiki to a project means it will be applied to all of the folders within that project.
You can also change the logo by adjusting "Look and Feel" settings at the project- or site-level