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 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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Identification technology