Calling card

A calling card was a card that individuals left behind as an indication that they had visited. Metaphorically, it could refer to any indication of another's presence.

In 2259, Ensign Nyota Uhura characterized a data chip left by the Kerkhovians on the shuttle Cervantes as a calling card, as it contained instructions on how to contact them via subspace. (SNW: "Charades")

In 2375, The Doctor, in his role as the President of Earth, informed photonic lifeforms invading the The Adventures of Captain Proton holoprogram that they had "dropped (their) calling card. I return it to you as a sign of good faith." (VOY: "Bride of Chaotica!")

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