Situation #1: You're in a private area. No detection skills, spells or abilities have revealed hidden characters, and no suspicious signs are present.
Rule: You cannot use spells, abilities, or items to detect hidden characters.
Situation #2: You're in a private area. Suddenly, a door opens and closes on its own, raising suspicion.
Rule: Offer an opt-out to the hidden character. If they don't accept the terms, you may initiate PvP actions.
Situation #3: You're in a public area and legitimately detect a hidden character.
Rule: Offer the hidden character an opt-out before any PvP action occurs.
Situation #4: You're in a private area and legitimately detect a hidden character.
Rule: The hidden character is automatically opted in for PvP, but strive to roleplay the situation first.