Searching

From Wikihack

Searching, which is used by pressing the [s] key, will have your character attempt to find adjacent hidden doors and traps. It also allows you to locate monsters that are invisible, hiding under items, clinging to the ceiling, or mimicing another object. Searching multiple times is more likely to get results. Rangers start the game with intrinsic automatic searching. Wielding Excalibur or wearing a ring of searching grants extrinsic automatic searching.