Detection and Hidden
When known opponents are Hidden, a character must attempt a Seek action to determine if their character knows where the opponent is located. If they fail, they cannot target the opponent with Actions. Alternatively, they might choose to Stride to look for the opponent, which might allow reveal them if the GM determines the opponent no longer has Cover from the character. However, if the opponent is Hidden due to Concealment or the lighting, simply moving will not reveal the opponent.
If an opponent’s presence is completely unknown, and they are undetected by the character, the character cannot even attempt a Seek action to reveal their presence. One cannot seek out what they are unaware exists in the first place. In some cases, the player’s character might not be aware an opponent exists, even if that character’s player does.