Hidden

Creatures that are hidden are invisible to other creatures. They cannot be interacted with, but can still be struck at if the player force attacks in that direction.

When a player character walks past a hidden creature, they search all adjacent tiles. A random number is rolled from 1 to the searcher's Intelligence, plus any additional bonuses to searching they may have. If this is greater than or equal to the hidden creature's difficulty, they will be revealed. This search can be performed multiple times by stepping back and forth until the hidden creature is revealed. True Kin characters can install a Penetrating radar implant to unerringly reveal any lurkers within its AOE.

Formula
Depending on the difficulty, the basic chance of successfully finding a hidden creature is:

Sewage eels
Sewage eels behave slightly different from other lurkers. Rather than existing but in hidden state, eels initially are nothing but a spawner, due to which they cannot be attacked until successfully revealed.