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Passive skills and command skills with the Permanent property apply their effects even if the unit is not present on the battlefield; whether it is off the table due to Ambusher or Scout, after it has been destroyed, or, in the case of a Character, after they join a unit (even if the skill is not present in their unit game profile).
Scout or Ambusher units with a Permanent ability, which are outside the battlefield, can decide to apply it (revealing its existence to the opponent) or not to do so (and remain hidden).