The cave spider, s, is a monster in NetHack. They are capable of hiding under items, taking some players by surprise; fortunately they are generally too weak to be a threat in this respect. Those generated along with a dungeon level always get a random item to hide under. Since spiders come in groups, the effect is something like a quasi-special room.
If you're waiting outside of a room full of monsters, e.g. a treasure zoo, and wondering why the monsters aren't coming out despite being awake, then it may be because a cave spider is hiding under an item in the doorway. The same is true for other monsters that hide under objects. This can be used to your advantage by dropping items in front of advancing cave spiders.