Secret Passages (also known as Alternate Routes and Geheime Wege in German) are hidden pathways, rooms and areas found throughout various worlds in the Metroid series, most notably Zebes. A staple of the series' gameplay, these passages often contain alternate routes or items that help Samus in her quest. These passages are not immediately visible when Samus enters a room, and are usually obstructed by Blocks. Secret Passages can be revealed using Bombs, beams (in Metroid: Zero Mission) or the X-Ray Scope (in Super Metroid).
Page 65 of the German Super Metroid guide features a section about these passages, grouped with sections about the other areas of Zebes. However, this cover refers to the abstract concept of Secret Passages in the game, rather than a separate area like Brinstar or Crateria.
German Guide (Translated)
"On the following pages the labyrinths of all sectors are displayed with secret passages, items and passageways. You should take a quick look at the maps only in emergencies, however."