Note that the unit may only use Conceal Self to sneak into provinces. The unit must be alone in order to conceal itself. It may not be stacked with any other characters, or possess any visible followers, such as peasants, soldiers, workers, etc., except for ninjas. It may also not be the first character inside a sublocation, building or ship. The owners of these small locations are always visible.
Hiding takes three days to accomplish.
However, turning off concealment takes zero time. A failure to gain entrance with MOVE will give the unit away to the structure's owner. Concealment does not hide noticeable actions such as attacking another unit, digging up a body, etc. Other than movement, actions which would generate a message in the location log will still generate a message. Hidden nobles may be spotted by other nobles using the SEEK command.