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. ()
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." ()
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Category:Identification technology