Since the movelist also gives enough information for tracking each piece that has moved, I will look into marking each of the opponent's pieces with a unique identifier, such as the space it began on.
I have now done that. Since this shows the locations of moved pieces, it somewhat diminishes the usefulness of the Spy. So I'm thinking of giving it the ability to detect enemy attacks by either not letting it move though check or not letting it move into check. I might also pair the Queen with the Decoy and the Spy with the Centaur or just make those extra options to leave the identity of the opponent's pieces more uncertain.
I have now done that. Since this shows the locations of moved pieces, it somewhat diminishes the usefulness of the Spy. So I'm thinking of giving it the ability to detect enemy attacks by either not letting it move though check or not letting it move into check. I might also pair the Queen with the Decoy and the Spy with the Centaur or just make those extra options to leave the identity of the opponent's pieces more uncertain.