Spoiler: Magic Rock
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It gives off an aura of Divination magic, not strong, but persistent and long lasting. It looks to be a variant of the Arcane Mark spell, one that can have its mark detection transferred or shared with another - in this case, the delivery birds, and can apply to more than one object, but likely has limitations elsewhere to offset this.

You remember reading about similar stones or other small trinkets being used in military strategy to give better overview of troop movements, but it looks like it might have been repurposed for these unsavoury means.