When activiated, other characters will not see the unit in their location reports, and will not see the unit come and go. The unit must be alone in order to conceal itself. It may not be stacked with any other characters, or possess any followers, such as peasants, soldiers, workers, etc. 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.