I never thought to look up what to do if I lost the SeaPass card in a port. Hope it is useful to others.
My aunt lost hers in Atlantis, and didn’t realize until we were on tour bus back to port. She freaked out and started crying(first cruise for her was very nervous) I headed straight to where they check ID and card, to ask what to do, they said to go to port security which was right outside.
Went and they checked her passport with ship manifest and let us through a side gate back to the ship. Once at gangway ship security supervisor came down to verify and let us through. Once we went through scanner we headed for guest services for a new card.